We say yajna means fire sacrifice, but that is not the actual meaning. Something done for the satisfaction of Lord according to a regulated (vaidika) method is a yajna. The srutis, the Vedas, say that yajna is one kind of manifestation of Visnu. Yajno vai Visnuh iti sruteh. This is the evidential verse in the Vedas. Srimad Bhagavat also says yajnaih sankirtana-prayair yajanti: we should perform worship through the yajna of sankirtan. If only there is a fire, then that is not a ful expression of yajna. The real fire of yajna is not a fire that burns visibly; it is a fire that burns our karma, and through that heavenly feelings arise within us.
You know the first verse of the Siksastakam. There, Mahaprabhu has given a very nice expression of His sankirtan-yajna.
anandambudhi-vardhanam prati-padam purnamrtasvadanam
sarvatma-snapanam param vijayate sri-krsna-sankirtanam
Chaitanya Mahaprabhu composed eight verses. He did not actually give much attention to composition; His verses, these eight verses, came out automatically from His lotus mouth. This is the first of them. He did not write any books but gave only these eight verses, maybe two or three others, but not more. His divine will was fulfilled by the six goswamis; they composed so much.
Hari-sankirtan first cleans our mind. The mind is a like mirror covered by so much dust. With it, we cannot see our own self. But when this mirror is cleaned by Hari-sankirtan, then we can see our own self. This is the first result that comes through Hari-sankirtan: cheto-darpana-marjanam.
In this material world, the jiva-souls are suffering from so many disturbances because of their karma. It is as though they are burning in a fire in the jungle. Sadness, unhappiness, attachment, detachment—many things come into the minds of the jiva-souls, and because of that they feel so much pain. Davagni means a fire in the jungle that arises automatically. Through sankirtan, that kind of pain goes away forever: bhava-mahadavagni-nirvapanam. That kind of fiery pain and disturbance within the jiva-soul’s mind and subtle body—all of it is removed.
Then, automatically, auspicious moonlight (chandralok) arises for the protection of the jiva-soul and brings joy to the heart of the jiva-soul. Such mercy comes, and everything becomes very peaceful and joyful. Sreyah-kairava-chandrika-vitaranam: sreyah means mangal [auspiciousness].
Moonlight automatically gives protection and joy to all living entities. The jiva-souls are burning because of their karma, but with the moonlight of Hari-sankirtan that goes away, peace comes into their minds, they get protection in their spiritual life. Then, pure love—pure beauty, attachment, and ecstasy—are revealed in the heart of jiva-soul, all for their Lord. Eternally feelings come to the jiva-soul, “The Lord is my master, the Lord is my husband, the Lord is my son, the Lord is my father. He is everything: the Lord is my life and soul. Without Him, I cannot live, and with Him, I will have full ecstasy and joy.” Through sankirtan, such feelings come to the heart of the jiva-soul.
Vidya-vadhu-jivanam: deeper meaning is also given inside this expression, that is, the feeling of madhura-rasa: the feeling of being newly married, the feeling of a willing and joyful marriage between husband and wife, comes permanently into the heart.
Then, everything in front of the jiva-soul becomes joyful, ecstatic, beautiful, and lovely. More and more, such feelings increase with Lord, and the jiva-soul becomes full satisfied. Purnamrtasvadanam: when someone drinks nectar, satisfaction must come into their heart, and the jiva-soul feels like that.
Then, Mahaprabhu says, “Param vijayate Sri-Krsna-sankirtanam: supremely it will get the victory, and we will live under its divine ecstatic shade, the divine merciful shade of Hari-sankirtan.”
This is expression of Chaitanya Mahaprabhu. He as the Yuga-avatar:
The religion in the Kali-yuga is to chant the Holy Name of Lord. Mahaprabhu Sri Chaitanyadev appeared, organised this, and brought joy to everyone’s heart.
Srila Avadhut Maharaj: Maharaj, Mahaprabhu also said, “I have no satisfaction, and I am not qualified to chant the Name.” Many devotees think about only the quantity of the Name and that some kind of easy practice of the Name is possible. Because the Name is advertised so much, they have taken the Name very cheaply. Can you explain Mahaprabhu’s feeling that we are not able to chant the Name?
Srila Govinda Maharaj: In His second verse, Mahaprabhu glorified Hari-nam, the Divine Name:
namnam akari bahudha nija-sarva-saktis tatrarpita
“All power is present in Your Name, and You have manifested so many Forms and Names: Rama, Krishna, Nrsimha, Vaman, Madhusudan, and so on. You have manifested many of Your Divine Names in this world by Your grace, and You have put all Your power in these Names. So, They are automatically powerful, but I am very unfortunate: I am chanting these Names, but the result of chanting Them is not coming to me. This is my very awkward condition.”
Then, with His next verse, Mahaprabhu explains how way we will get the proper result in chanting:
trnad api sunichena taror iva sahisnuna
amanina manadena kirtaniyah sada harih
If you want to get the proper result, you must be like this. You first have to control your environment, which is always giving you trouble. Feel your situation as much as possible: feel how much you can do for yourself. Then, automatically, humility will come to you. Controlling your environment means to not make anyone your enemy. Then, nothing will go against you, and feelings, real feelings, will come into your heart. This is the basis of the three qualities given by Mahaprabhu to chant the mahamantra.
Srila Bhakti Vinod Thakur translated this third verse of Mahaprabhu into Bengali. The meaning is that you must be humble, be tolerant, and give honour to others. Then, quality will come to you. With that good quality, you can chant the Holy Name of Lord, and that chanting will be fruitful. So, be humble, be tolerant, and give honour to others. This is first condition. And inside that, there are more disturbances to chanting, that is, the ten kinds of offences. We always give everyone a small book where these ten kind of offences are written. If you avoid the offences and chant the Holy Name, the proper result must come to you.
Spoken in St. Petersburg, Russia.
Translation of the third verse of Sri Siksastakam by Srila Bhakti Vinod Thakur:
sri-krsna-kirtane yadi manasa tohara
parama yatane tahi labha adhikara 
yadi–If tohara [achchhe]–you have manasa–a mind kirtane–to chant sri-krsna–the Name of Sri Krsna, [then with] parama–great yatane–care, labha–get adhikara–the qualification tahi–for that. 
If you aspire to chant the Name of Krsna, then, with great care, become qualified to do so.
trnadhika hina, dina, akinchana, chhara
apane manabi sada chhadi’ ahankara 
chhadi’–Give up [your] ahankara–false ego, [and] sada–always manabi–consider apane–yourself hina–lower adhika–than trna–grass, dina–poor, akinchana–needy, [and] chhara–mean. 
Give up your false ego, and always consider yourself lower than grass, poor, needy, and mean.
vrksa-sama ksama-guna karabi sadhana
pratihimsa tyaji’ anye karabi palana 
sadhana karabi–Practise guna–the quality ksama–of tolerance sama–like vrksa–a tree. tyaji’–Give up pratihimsa–vengeance [and] palana karabi–nurture anye–others. 
Practise tolerance like a tree. Give up vengeance and nurture others.
jivana-nirvahe ane udvega na dibe
para-upakare nija-sukha pasaribe 
dibe na–Do not cause udvega–anxiety ane–to others jivana-nirvahe–for the maintenance of [your] life, [and] pasaribe–forget [your] nija–own sukha–happiness upakare–in the course of helping para–others. 
Do not trouble others for your own upkeep, and forget your own happiness in the course of helping others.
ha-ileo sarva-gune guni mahasaya
pratisthasa chhadi’ kara amani hrdaya 
ha-ileo–Even if you are mahasaya–honourable [and] guni–qualified [with] sarva–all gune–qualities, chhadi’–give up asa–desire pratistha–for prestige [and] kara–make [your] hrdaya–heart amani–prideless. 
Even if you are honourable and qualified with all qualities, give up desire for prestige and make heart your prideless.
krsna-adhisthana sarva-jive jani’ sada
karabi sammana sabe adare sarvada 
jani’–Knowing [that] sarva–every jive–soul [is] sada–eternally adhisthana–an abode krsna–of Krsna, sarvada–always adare–respectfully sammana karabi–honour sabe–everyone. 
Knowing Krishna resides eternally within every soul, always respectfully honour everyone.
dainya, daya, anye mana, pratistha-varjana
chari gune guni hai’ karaha kirtana 
dainya–Humility, daya–compassion, mana–honouring anye–others, [and] varjana–giving up pratistha–prestige— hai’–become guni–qualified [with these] chari–four gune–qualities [and] kirtana karaha–chant. 
Humility, compassion, honouring others, and giving up prestige—become qualified with these four qualities and chant the Name.
bhakati-vinoda kadi’ bale prabhu-paya
“heno adhikara kabe dibe he amaya” 
kadi’–Crying paya–at the feet prabhu–of the Lord, bhakati-vinoda–Bhakti Vinod bale–says, “he–“O kabe–when dibe–will You give amaya–me hena–these adhikara”–qualifications?” 
Crying at the feet of the Lord, Bhakti Vinod says, “O when will You give me these qualifications?”