Chapter 4: Renunciation of Action; Jnana Karma Sanyasa Yoga
In this chapter Krishna begins with detailing the roots of Sanatan Dharma and Yogic Teachings and details the idea or Avataras or incarnations. An incarnation is Parmatman or Supreme Self in human body, like Krishna himself to establish humanity and Dharma or righteousness in society. 
Further Krishna explains how Atman or oneself can be totally free of all the actions and remain unbounded of karma, time and space. Krishna also explains reality at various levels like empirical reality (the sun, moon, stars, birds, trees, etc), the subjective reality (individual perception and experiences), Ultimate reality or Brahman.

Verse 24: The means of offering is Brahman. The oblation is Brahman, offered by Brahman, into the fire, which is Brahman. Brahman is indeed to be reached by him who sees everything as Brahman.