This is an extra security mechanism used in unix like OS for accessing any file in the system.it is also like proc file system.
To check the status u can use command sestatus -v .It will show the status whether it is Disabled/ Permissive/Enforced Mode.
Disabled mode - When any application wants to use any file, it will not take extra security care and gives access to use it.
Enforced Mode - Here the application sends a request to selinux security server and it checks access vector cache (AVC), where subject and object permissions are cached previously.
Permissive mode - Here It dont enforce policy but logs the warning messages about the deny of permission.
so using the configuration files present in /selinux we can change the modes of operation.
echo 0 > /selinux/inforce - Permissive Mode
echo 1 > /selinux/inforce - Enforced Mode
But all changes is temporary so it changes after restart the system to default.
To make it permanent
we can change in the /etc/selinux/config file
SELINUX=disabled or enabled
For more you can also refer explained beautifully