This goes on the Bottom Board and makes the entrance to the hive smaller. The bottom board is just a doorway, it is no door. If your house had no door you would probably guard the doorway to your house so that not just anyone could walk into your house but if your doorway was very big you might have trouble guarding it. This is the same with the bees, if the “doorway” is too big for a smaller colony to guard they will struggle so don’t hesitate to use an entrance reducer.
