To lighten something is to make it brighter or shine a light on it. You can lighten your dark basement room with plenty of lamps and a coat of white paint.
Besides "make lighter or illuminate," another meaning of lighten is "make lighter in weight." So you can lighten your load by taking all the heavy books out of your backpack. Figuratively, "to lighten" is to be more cheerful or relaxed, as in: "She decided to lighten the somber mood in the room by telling a few of her funniest knock-knock jokes."