The modulo (or "modulus" or "mod") is the remainder after dividing one number by another.
Example: 100 mod 9 equals 1
Because 1009 = 11 with a remainder of 1
12 Hour Time
12-hour time uses modulo 12
Example: 14 mod 12 equals 2
Because 1412 = 1 with a remainder of 2
So 14 o'clock becomes 2 o'clock.
We are only interested in where we end up, not how many times around.
Play with it here:
We normally write "mod": 14 mod 12 = 2
Some programming languages use the percent sign: 14 % 12 = 2, but % is just a convenient symbol and has nothing to do with percents.
Modulus and Addition
You can also play with adding numbers using different modulo values here:
Modulus with Multiplication
You can also have a play with this. It multiplies each number by your chosen value, then links to the modulus of that multiplication. It makes some really interesting patterns: