What is SELinux?
The Security-Enhanced Linux is an extension to the Linux Kernel, made by the NSA (National Security Agency). It implements Mandatory Access Controls (MAC), which allow an administrator to define, how applications and users can access resources on a system.
There is more detail in the SELinux Wki: https://selinuxproject.org/page/FAQ
… and the CentOS documentation: https://wiki.centos.org/HowTos/SELinux
Some distributions have it installed by default, but not active, some have it …