Queen excluders keep the queen away from specific Boxes. They are usually a metal or plastic grid (sometimes with a wooden frame around the edges) that is tight enough to keep the queen out but let the Worker Bees through. Note: Drones also cannot get through the queen excluder so if you use one you will need an upper entrance to let the drones out. You can do this by using an Inner Cover with a top entrance in it of by propping open your Outer Cover with wooden door shims.
Usually queen excluders are used to keep the queen out of Honey Supers so that she doesn’t lay eggs in the honey you want to Extract.