what are modulus?

Answer 1

the modulus ( %) is an operator that returns the remainder of a division .

example : 7%5== 2 . 2 is the remainder .

Answer 2

Pretty much what bahhax404 said, it's the remainder from division, here's an example.

int a = 10;
int b = 3;
sum = 10%3;

cout << "The remainder of this is:" << endl << sum;

Output:
The remainder of this is:
1

3 goes into 10 3 times before it becomes a decimal. So, since it went in 3 times that's 9, and 10 - 9 = 1.
Hope this helped !