In a word: consciousness. The theme of the book, and of Hofstadter’s work in general, is the way objects can exist at different levels of description and abstraction, and that those levels of description can interact with one another in complex ways he calls ‘strange loops’.

Probably the key section, which illustrates all these ideas most clearly, is the two-part dialogue Prelude, Ant Fugue which appears at the start of Part II. Hofstadter uses the analogy of a conscious ant colony, in which the seemingly independent action of individual ants, communicating through mutual exchanging of scents and other signals, combines to create larger-scale processes that can be interpreted as the thoughts and feelings of the colony as a whole.

You can read this dialogue (slightly poorly transcribed) here - it was reprinted in Hofstadter and Dennett’s compilation of philosophical writings about consciousness The Mind’s I. Chapter 11: Prelude . . . Ant Fugue