There are four OCTAL (0..7) digits, which control the file permissions. But often, only three are used. If you use 600 it equals 0600. The missing digit is appended at the beginning of the number.
Each of three digits described permissions. Position in the number defines to which group permissions do apply!
1 – can execute 2 – can write 4 – can read
The octal number is the sum of those free permissions, i.e.