When you redecorate something, you renovate or fix it up again. If, for example, you redecorate your bedroom, you might get a new comforter, move your bed across the room, and hang some pictures on the wall.
When people redecorate, they may paint walls entirely new colors or put up wallpaper. They might also hang new curtains, pull up old carpet and refinish floors, rearrange furniture, and even buy brand new furnishings. This verb almost always applies to a home, like an apartment, house, or dorm room. Redecorate adds the "again" prefix re- to decorate, which has the Latin root decus, "an ornament."