In math, subtraction is when you take one number away from another number. In other words, the subtraction of two from five gives you an answer of three.
In school, subtraction is typically the second operation you learn in arithmetic, after addition. When a problem has you take four apples away from eight apples, that's subtraction. If you see the word minus, or the symbol -, that's also subtraction. The word itself comes from the Late Latin subtractionem, "a drawing back or taking away," from subtrahere, "take away," which combines sub, "from under," and trahere, "to pull."