Security-Enhanced Linux is a Linux optional feature ("kernel module") that provides mechanisms for protecting parts of the operating system via "mandatory access controls", aka MACs. It was created by Red Hat and the US National Security Agency, and first released in December 2000.
It is a standard part of Android (since version 4.3) which makes sense given that a mobile phone used by a member of the public is a rather threatening security environment, compared to the 1970s computers with skilled system administrators where the basic UNIX security system was developed.
More details: https://source.android.com/security/selinux/
