What Is Remainder?
A remainder is the amount left over when one whole number doesn't divide evenly into another. It's always smaller than the number you're dividing by (the divisor) — if it weren't, you could fit at least one more whole group in.
Example
17 ÷ 5 = 3 remainder 2, since 5 fits into 17 three whole times (15), leaving 2 left over.