A queen excluder serves as a selective barrier inside the hive, limiting where the queen and drones can travel. This metal queen excluder is placed over the brood chamber. The screen is wide enough to let the workers through, but too narrow for the queen.
Hint: If you are concerned that your excluder will slow your bees down when storing nectar in the supers, just turn it sideways. The bees can go around the edges, but the queen USUALLY stays in the middle of the hive and is excluded. Once you are confident your supers are drawing enough workers, you can turn the excluder to its traditional placement, but remember to check for your queen up in the supers!