The kern directory holds the core functionality of FreeBSD: system startup, core system calls and syscall dispatching, process management, scheduling, signals, file descriptors, top-level filesystem code, interprocess communication and terminal handling.

The 4.3BSD Internals book by Leffler, McKusick, Karels and Quarterman covers kern in:

Chapter 2, pp 19-42 Overview of the Kernel.
Chapter 3, pp 43-66 Kernel Services.
Chapter 4, pp 69-108 Process Management.
Chapter 6, pp 169-186 I/O System Overview.
Chapter 9, pp 259-278 Terminal Handling.
Chapter 10, pp 281-309 Interprocess Communication.
Chapter 13, pp 393-411 System Startup.