This is a convenient shorthand to calculate the remainder when one integer is divided by another and assign it back to the variable the calculation was done on.
x %= divisor; // equivalent to the expression x = x % divisor;
x: variable. Allowed data types: int.divisor: non zero variable or constant. Allowed data types: int. int x = 7;
x %= 5; // x now contains 2
x %= 10 will not always be between 0 and 9 if x can be negative.