The expression "a - b" represents the mathematical operation of subtraction, where "a" is the minuend and "b" is the subtrahend. This operation calculates the difference between the two values, resulting in a new value that indicates how much "b" is less than "a." In practical terms, if you have 5 apples (a) and you give away 2 apples (b), the calculation would be 5 - 2, resulting in 3 apples remaining. Subtraction is a fundamental arithmetic operation used in various fields, including finance, science, and everyday problem-solving.