A clock shows hours 1 through 12. After 12 comes 1 again. If it is 10 o'clock and you wait 5 hours, the clock shows 3, not 15. The numbers wrap around in a cycle.
This wrapping is modular arithmetic. You divide 15 by 12, get quotient 1 and remainder 3. The remainder is what the clock shows. The quotient tells you how many times you went around the clock.
In math we write 15 mod 12 = 3. The mod operator gives you the remainder after division. This same pattern applies to any cyclic system, not just clocks.