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/common			source code for stuff (libraries) that can run in kernel or user land
src/common/inc		user&kernel-shared headers
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