karma brahmodbhavam viddhi brahmaaksharasamudhbhavam |
tasmaatsarvagatam brahma nityam yajne pratishthitam || 15 ||

Know that actions arise out of the Vedas, and that the Vedas arise out of the imperishable (eternal essence). Therefore, yajna is always established in that all-pervading eternal essence.

karma : actions
brahmodbhavam : arise out of the Vedas
viddhi : know
brahma : Vedas
aksharasamudhbhavam : arise out of the imperishable
tasmaat : therefore
sarvagatam : all-pervading
brahma : eternal essence
nityam : always
yajne : yajna
pratishthitam : established

Shri Krishna reveals the connection between yajna and the eternal essence in this shloka. Here he says that yajna is an integral part of the eternal essence, because it is born out of the Vedas. Symbolically, the Vedas represent the “rule-book” of the universe, and so yajna becomes an essential law embedded in the universe.

In the second chapter, which was an overview of the entire Gita, Shri Krishna pointed Arjuna to the ultimate goal of life – realization of the eternal essence. In this chapter, Shri Krishna connect karmayoga to the eternal essence. He explains that karmayoga is a means, a technique, to realize the eternal essence, because the eternal essence created the rules of the universe.

So in summary, the teaching of karmayoga so far is : Actions are of two types,  selfless and selfish. Selfish actions bind us to wordly objects, and selfless actions do not. Therefore, Shri Krishna urges us to move gradually from the level of inaction to selfish action first, and then from selfish action to selfless action or yajna. Only through acting in the spirit of yajna will we move forward on the path to realizing the eternal essence.