SELinux is a security enhancement to Linux which provides users and administrators a more granular, powerful access control mechanism. It consists of a kernel component which enforces the security policy, a set of userspace tools to manage and manipulate SELinux security policies, and the SELinux security policies themselves.
The main Linux Kernel README can be found at Documentation/admin-guide/README.rst
The canonical SELinux kernel repository is hosted by kernel.org:
There is also an officially maintained GitHub mirror:
The SELinux reference policy, userspace tools, and test suites are hosted by GitHub: