To repair means to fix or mend something so that it is in good working order again. If you forget to put oil in your car and you blow the engine, it's going to cost you an awful lot to repair it.
Not only does repair describe the act of fixing something, it can also be used as a noun for the fixing itself, as in “Who did the repair on this house? It looks terrible!” In a totally different meaning, if you repair somewhere, you return back to or go to a place for rest or shelter. If you're outside and it begins to rain, you might want to repair to the house where it's dry.