If I run
it gives me this output:
-rwsr-sr-x 1 root root 315432 Nov 10 2007 /usr/bin/crontab
What does 's' means in the permission mask?
Why do they need to use that?
The s means setuid (or setgid). In this case the crontab program will run with root permissions (because root is the owner) when ran by other users.
These are permissions as listed with the ls command.