To remediate is to correct or make right. If you accidentally ran over your neighbor's bike with your car, you could remediate the bad situation by paying for the bike's repair.
When you remediate some kind of damage or mistake, you repair it or set it straight. To remediate often means to make up for a lack of something, like when schools remediate students who have struggled in certain classes by giving them extra instruction after school. The origin of remediate goes back to the Latin word remedium, a cure, remedy, or medicine.