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So basically the modulo operator does this:
You have 2 numbers, when you divide one of them on the other, the remainder (that is the number left before you start making decimals) will be the result of the modulo.
So imagine you want to divide 13 over 3
there are 4 threes in 13 and an extra one (4*3 = 12, 12 + 1 = 13)
So 13 % 3 = 1 (which is the remainder)
Hope this helps 
if you need any more explanation, then feel free to @ me