SELinux (Security-Enhanced Linux) is a mandatory access control layer that sits on top of the traditional Unix rwx permission model. Regular permissions decide "can this user read that file?" — SELinux decides "can this process, running under this type label, access that resource, which has its own type label?" Every file, process, port, and socket on the system has a security label (technically a security context), and the kernel consults an in-memory policy to allow or deny each access. On the RHCSA exam, SELinux is guaranteed to come up — you'll need to know how to toggle enforcement, relabel files, fix mislabeled web/database content, and set booleans and ports.
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