The expression "number % 2" is a mathematical operation known as the modulus or remainder operation. It calculates the remainder when a given number is divided by 2. For example, if you take the number 5, dividing it by 2 gives a quotient of 2 and a remainder of 1. Thus, 5 % 2 equals 1. This operation is commonly used to determine if a number is even or odd. If the result of "number % 2" is 0, the number is even, meaning it can be divided by 2 without a remainder. Conversely, if the result is 1, the number is odd.