Yajna’ is derived from the Sanskrit root ‘yaj’ meaning to worship. The act of performing yajna is the act of offering worship to the gods or to the supreme being.
There were 3 main categories of yajna – pAkayajna, haviryajna and somayajna, each of which have several tens of individual yajnas. pAkayajna involved the use of cooked food offerings made from grains, fruit, vegetables, yogurt, ghee, etc. haviryajna involved use of raw offerings – uncooked rice and other grains, milk, honey and other food stuff. pAkayajnas and haviryajnas were done in frequency ranging from several time a day (agnihotra) to daily, to monthly (darshapUrNamAsa), bi-monthly, half-yearly, etc. Somayajna consists of offering the Soma juice, along with other items. Animal sacrifice was optionally done only as part of the biggest Soma yajnas, such as atirAtra, agniShToma, vAjapeya, sautrAmaNi, ashvamedha and rAjasUya. The last 4 were done only by Kshatriyas.
The big yajnas were very public events as they are even today. The yajamAna (performer of yajna) needs to be married, and needs to have maintained his sacred fire (gArhapatya) which he received from the sacred fire of his wedding. The fire for the yajna is taken from his gArhapatya. Before he can begin, he needs to repay all his debts or get permission from the lenders to do the yajna, and ask for elders of the community to give their agreement and blessings. The entire village or community will get involved, because of the various accessories needed. The potter will need to make the bricks to construct the vedi and make all the various pots and vessels needed. The carpenter is needed to build the yajna-shAlA, and so on.
Now coming to the symbolism of the yajna. In the Vedas, the primordial state is of non-existence. “Darkness covered darkness… everything was unfathomable ‘water’…. that One breathed without breath by its very nature…” (RV10.129). The first ray of existence emerged when Agni was kindled. This concept is described as an allegorical story in RV 10.51 and 10.124 where the Devas are pleading with Agni to come out of the darkness so that the yajna can begin. Then once the yajna has begun, what can be offered into it? What else is there except Agni himself? This is why it is frequently said, “Agni – offer yourself to yourself, or offer to your own house, or grow your own body”. How can this be? According to Vishvamitra (RV 3.26.7) Agni is “both matter and energy”, so the grains that are offered are already Agni’s form, so basically we are adding Agni to Agni. The same event can be found in Purusha Suktam (RV10.90) where Purusha is the only existing entity so he is himself offered into the yajna, and the outcome is his divisions become the parts of the universe. So in summary, the first yajna was the act of creating the universe. As an act of reverence to this mystical creation, the yajna is performed by human beings, as a way to emulate the gods. Each day, each fortnight, each month, each season, each year are all mini-creations. So by performing a yajna at each new time division, we are emulating the macro-creation and immersing ourselves in the metaphysical truth of one existence, consciousness and bliss.
A special yajna called agni-cayana (gathering or building up of Agni) is a long yajna that lasts several days and nights continuously. The symbolism here is that Agni or Purusha or Prajapati who was divided up into the universe during creation is mystically being rejoined and made whole again.
So you see, originally yajna had the role of a meditation accessory. Like rosary beads, or pictures or idols. Performing the yajna was like enacting a play, only this play is meant to help us understand and meditate (manana, nididhyAsana) on the metaphysical truth behind existence.