variable = rand() % max;

rand() produces a random integer value.
% is called the modulus operator. It's basically what you learned to be the remainder in division. Ie. 5 % 3 = 2 and 16 % 5 = 1
As for this function, it actually only returns a number from 0 to max - 1, as you can't divide a number by a number and have the remainder equal to the divisor. Ie. a % b cannot equal b.