The modulus operator % in C is used to find the remainder of the division between two numbers. It is an arithmetic operator that returns the remainder when the first operand is divided by the second operand. For example, in the expression 17 % 5, the number 17 is divided by 5, and the remainder of this division is 2. So, the result of 17 % 5 is 2. The modulus operator is useful in many situations, such as determining if a number is even or odd. For instance, num % 2 == 0 checks if a number is even, as the remainder of dividing an even number by 2 is always 0.