Why does using this code can generate a random password?

Here a snippet for generating password code, I have 2 questions about this, Could you please share how to understand?

1. urandom(6), help from urandom said,return n random bytes suitable for cryptographic use, it is say, it will return 6 bytes, is it 6 of ASCII ?

2. ord(c) , get the decimal base for above bytes, why here transfer to decimal base?

Help for urandom:

def urandom(n): # real signature unknown; restored from __doc__
    """
    urandom(n) -> str

    Return n random bytes suitable for cryptographic use.
    """
    return ""

python script:

from os import urandom
letters = "ABCDEFGHJKLMNPRSTUVWXYZ"
password = "".join(letters[ord(c) % len(letters)] for c in urandom(6))