An entrance reducer is simply a barrier placed at the entrance to a beehive that reduces the size of the opening. They are usually made of wood, but can also be made from metal or plastic.
Adding an entrance reducer is a good idea to help the bees in a hive protect itself from robbing honey bees or from other pests entering the hive (like a mouse). The smaller entrance makes guarding easier and gives a smaller or weak hive a fighting chance. It is generally recommended to add an entrance reducer when a nuc or package of bees is installed as they are in a weakened state.
However, when should the entrance reducer be removed?
Until nectar is plentiful keep the entrance reducers on the weaker hives and fully open for only those hives you know to be strong and populous. When nectar becomes more available weaker colonies expand quickly and soon are able to defend their hives. At that point, you can enlarge their entrances or remove the reducers altogether. There are many reasons to use an entrance reducer, although they are almost never used at the height of a nectar flow.