Yajna means Vishnu.

In Bhagavad-gita it is clearly stated that there are many types of yajna performances recommended in the Vedic literatures, but actually all of them are meant for satisfying the Supreme Lord. Yajna means Vishnu. In the Third Chapter of Bhagavad-gita it is clearly stated that one should work only for satisfying Yajna, or Vishnu. The perfectional form of human civilization, known as varnasrama-dharma, is specifically meant for satisfying Vishnu. Therefore, Krishna says, “I am the enjoyer of all sacrifices because I am the supreme master.” However, less intelligent persons, without knowing this fact, worship demigods for temporary benefit. Therefore they fall down to material existence and do not achieve the desired goal of life. If, however, anyone has any material desire to be fulfilled, he had better pray for it to the Supreme Lord (although that is not pure devotion), and he will thus achieve the desired result.

Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Seventh Canto, Chapter 03 – Text 24