This file explains all the directories of the source tree :

doc					documentation about the system
mnt					temporary folder for mounting disk images
src					source code
src/kernel			source code for everything running in kernel-land
src/include			headers that the kernel and the userland must share
src/user			source code for stuff that can run only in user land
src/user/lib		userland libraries
src/user/lib/inc	header files for these libraries
src/user/app		userland apps