The mention command does a very simple thing. Actually with it you set the default permissions you want for files. It could be for every created file in the system or for a specific user session.
If you for example want for every single file only read permissions then you have to do this:
777
(all permissions) - 444
(read permissions) = 333, the umask for only readable permissions is: 333
Have a look here:
Setting default permissions