An amount left over after division (happens when the first number does not divide exactly by the other).
Example: 19 cannot be divided exactly by 5.
The closest you can get without going over is 3 x 5 = 15, which is 4 less than 19.
So 4 is the remainder
So the answer of 19 ÷ 5 is "3 with a remainder of 4", which means that 19 can be divided by 5 into 3 parts but with 4 left over, and is usually written "3 R 4".
Play with it below, there will often be a remainder: