I don't understand what the % do?

Answer 1

% is a modulus operator, and it is used to return the remainder of a division.

For example:

5 % 2 = 1
10 % 5 = 0
30 % 7 = 2

A very common, and beginner way to use the modulus is to tell whether or not a number is even:

if (x % 2 == 0)
{
//even
}
else
{
//odd
}

Answer 2

25:3=8 rest 1
explication:
25/3=8
25%3=1

Answer 3

it returns the remainder

Answer 4

Yeah, all it does is to return the remainder.