Unix file permissions are written as three octal digits.
Each permission is the sum of read(4), write(2) and execute(1), giving one digit (0-7). For example, read+write+execute = 7, and read+execute = 5.
The three digits are the permissions for owner, group and others. For example, chmod 755 = owner rwx(7), group r-x(5), others r-x(5).
Because three bits map exactly to one octal digit. Use this tool to convert between octal and binary and inspect the permission bits.