When you rectify something, you fix it or make it right. Some English teachers will give you a chance to rectify any mistakes you've made in an essay and hand in a second, edited draft.
This verb often turns up in sentences like "The truce should help to rectify this situation between the two countries," or "I'd like to rectify the misspellings in this article before I print it." To rectify is to correct or improve something — you can imagine straightening out something that's bent to make it work better. The Old French rectifier, in fact, means "to make straight," from the Latin root rectus, "straight."