What is the modulus number of a counter?

The number of unique states that a counter may have before the sequence repeats itself is the modulus of the counter. Example, Modulus 10 would have the counter with states 0-9 and then reset to zero.
A modulus counter is a system of counting when the division of a variable number (x) by a set fixed number (y) does not leave a remainder.

For example:

x%y=0;

Suppose y=2 and x is variable:

1%2=1;

2%2=0;

3%2=1;

4%2=0;

5%2=1;

6%2=0;

....

If y=5:

1%5=1;

2%5=2;

3%5=3;

4%5=4;

5%5=0;

6%5=1;

....

So basically it's a counter that skips a certain number of steps (y) that you set.