A remainder is the amount left over when one number cannot be divided exactly by another number. It's the part that doesn't fit evenly into the groups you're making.
If you have 7 cookies and you want to share them equally among 3 friends, each friend gets 2 cookies. There will be 1 cookie left over. That "1" is the remainder. So, 7 divided by 3 is 2 with a remainder of 1.