The document discusses Go's lightweight scheduler. It introduces Go routines and explains that Go's scheduler runs within the application rather than relying on the OS scheduler. It describes the basic structures used including M (threads), P (contexts), and G (Go routines). It then explains how the scheduler is initialized, how new Go routines are created and scheduled, and how scheduling occurs in response to system calls or other triggers. Source code locations are provided and diagrams illustrate the relationships between threads, contexts, and Go routines.