Owner, Group, and World in that order stand for the numbers 4 0 and 3
When translated to binary this is what the numbers mean
r w x
4 -- 1 0 0
0 -- 0 0 0
3 -- 0 1 0
r -- read
w -- write
x -- execute
If its a zero, means you don't have the permission to do that particular task.
So, that means is theres a 0 in the r column you cannot read that file.
As you can see from the r column in a 403 code, NO one has permission to read it except the owner (4). The group(0) and the rest of the world(3) both are denied access to this particular file.
