Chapter eight begins with Arjuna asking seven questions in two verses, which is the densest interrogation in the book. What is the absolute? What is the self? What is action? What is meant by the ground of the elements, and by the ground of the gods? Who is the ground of sacrifice in this body, and how? And how are you to be known at the time of departure by those who are disciplined?
The replies are given in the same compressed style. The absolute is the imperishable and supreme; the self is the essential nature of each being; action is the creative force that gives rise to beings; the ground of the elements is perishable existence; the ground of the gods is the primal person; and the ground of sacrifice is Krishna himself, here in the body. Six answers in three verses.
This makes chapter eight the most technical stretch of the Gita, and the most dependent on the vocabulary of the older Upanishadic literature that the book inherits. A reader coming to the Gita for its ethics tends to skip it, and a reader coming from Vedanta finds it indispensable, because these definitions are the terms every later commentator argues in.
The seventh question, though, is not technical at all, and the rest of the chapter belongs to it. Having established what the terms mean, the text turns to a single practical matter: what a person is doing at the moment their life ends, and what determines it. That question, rather than the definitions, is why the chapter is recited at deathbeds.