The remainder is the amount left over after division.
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, usually written "3 R 4".
There are four ways your kiddo has learned to interpret the remainder. They are drop it, add it, share it, and use it.