Permissions:

1 – can execute
2 – can write
4 – can read

The octal number is the sum of those free permissions, i.e.

3 (1+2) – can execute and write
6 (2+4) – can write and read

Position of the digit in value:

1 – what owner can
2 – what users in the file group(class) can
3 – what users not in the file group(class) can

Examples:

chmod 600 file – owner can read and write
chmod 700 file – owner can read, write and execute
chmod 666 file – all can read and write
chmod 777 file – all can read, write and execute