Modulo

A mathematical operation, defined as the remainder after an integer division.

Thus a mod b = 'the smallest positive remainder of a' after subtracting as many times b as possible.

Examples:

15 mod 4 = 3 since 15-3*4 = 3
42 mod 9 = 6 since 42-4*9 = 6
15 mod 5 = 0 since 15-3*5 = 0, etc ...

In image processing, to say that a resolution is mod-n means that the resolution is exactly divisible by n, i.e. res mod n = 0.