The expression "i % 2" is a mathematical operation that calculates the remainder when the integer i is divided by 2. The result of this operation can only be 0 or 1. If i is an even number, the remainder is 0, indicating that it is divisible by 2. If i is an odd number, the remainder is 1, showing that it is not divisible by 2. This operation is commonly used in programming and computer science to determine the parity of a number. Parity refers to whether a number is even or odd, which can be useful in various algorithms and data structures.