Answer
The word Yajna means worship or adoration. However, its specific traditional meaning is to worship or propitiate the Devas by offering oblations into the sacred fire (Agni). This, however, is the Vedic meaning of Yajna.
In the Rigveda, Agni is called the mouth of the Devas, and the belief was that whatever is offered into the fire in the name of a particular deity would reach that deity.
Thus, in the Vedic system of worship, the fire sacrifice (Agni Yajna) was the primary method of worshipping the Devas and adoring God.
But as you know, how much expense can be incurred in performing even a single fire sacrifice — even without a priest. If conducted through a priest, the priest's dakshina (honorarium) must also be given. A Yajna requires ghee, sesame seeds, rice, and other materials as havishya (oblation substances). Therefore, a poor person could not perform fire sacrifices on a regular basis. This was a very significant problem.
In the Vedic way of life, the Agnihotra was prescribed at least twice daily, which also required making offerings into the fire; this was mandated according to Manusmriti. Manusmriti even states that in truth, the sacred fire should be kept burning continuously in the household — and this perpetual fire was called 'Ajasra.' Now imagine how much firewood would be required, how much labor, and in that era, how much effort went into gathering firewood from the forests. Therefore, the Bhagavad Gita did not declare fire sacrifices as the primary or mandatory component of God's worship; rather, it set them aside. Fire sacrifices are also called dravya yajna (material sacrifice) because they required dravya (material substances). For this reason, the Lord described ten to twelve types of Yajna in the fourth chapter of the Gita. And for the worship of God, He went so far as to say that He is supremely pleased even with a leaf, a flower, a fruit, or water — if offered with sincere devotion. In saying so, He was also implicitly addressing the burden of expense in Vedic sacrifices. Then, at one point, He declared: 'Among all yajnas, I am Japa Yajna' — meaning He designated chanting as a form of Yajna and placed it among the most exalted of sacrifices. Now consider — for chanting, you do not even need leaves, flowers, or fruits. Furthermore, He explicitly stated that Jnana Yajna (the sacrifice of knowledge) is superior to Dravya Yajna (material sacrifice), and He identified the study of the Gita itself as Jnana Yajna. Note that neither chanting nor studying the Gita costs any money. In this way, He took spirituality out of the exclusive domain of the wealthy and made it accessible and easy for all.
If you study the fourth chapter of the Gita, the remaining types of Yajna that He describes will become clear. Among them are Deva Yajna (worship of the divine), Brahma Yajna (sacrifice of sacred knowledge), Atma-Samyama Yajna (the sacrifice of self-restraint), Pranayama Yajna (the sacrifice of breath control), and several others.
So the general meaning of Yajna is the worship of God, by whatever means. And without the worship of God, moksha (liberation) is not possible. Therefore, Yajna is indispensable.