Yajna, the fire sacrifice, is the central ritual of the Vedic sacrificial cult. The basic idea of Yajna according to the Vedic ritualists was that man can have a happy and prosperous life only if he lived in harmony with his environment, consisting of Nature and the Devas who control the forces of Nature. Man gets his progeny and his sustenance as the gifts of Nature and he should express his thanks to the Devas. Man is required to make an offering of thanks-giving to the Devas a share of the good things of Nature which he gets by their goodwill. This offering is made through fire which is the link between man and the Devas. This thanks-giving takes the form of ritualistic fire sacrifices with offerings of commodities and utterance of Vedic hymns. Proper performance of these Yajnas or sacrifices secures the goodwill of the Devas through which man’s survival and sustenance is assured. Later on in Chapter 4 the Gita itself speaks of other Yajnas (Dravya-yajna, Pranayama-yajna, Tapo-yajna, Svadhyaya-yajna and Jnana-Yajna) of which the fire sacrifice is only one. Thus in the Gita view Yajna ultimately is a symbol for all the moral and spiritual effort of man for his higher evolution.
Adopted from Swami Tapasyananda’s commentary on the Gita.