What does the permissions number represent?

[Kyojii]

For example when it says a users permissions are "16777215".

[kh99]

If you represent the value in binary, each bit would be a "flag" that indicates whether or not the user has that permission. The meaning of each bit depends on where exactly you found that value. (That value translates to binary of 111111111111111111111111, which seems a little strange because they're all set).