Read, write, and execute are represented by the numbers 4, 2, and 1, respectively, and you add them together to get the permission
Maybe I’m the weird one here but this seems like a counter intuitive way to remeber/explain it. Each octal digit in the three digit number is actually just 3 binary digits ( 3 bit flags) in order of rwx. For example read and execute would be 101 -> 5.