When you make a revision, you go about editing or rethinking whatever it is you're working on. So that paper on Donald Duck's influence in national politics? It could probably use a revision or two.
The word revision is made up of re, as in "repeat" or "redo," and vision. So you can think of a revision as a redo of your original vision. It's related to the word revise, which is the act of making changes to something original. Just about anything can get revised, from a script that needs rewrites to a wardrobe with some questionable pieces.