In beekeeping, the 'queen excluder' is a barrier that's placed inside the beehive between the brood box (where the queen lays her eggs) and the honey super (the honey storage box). The slits in the queen excluder allow worker bees (the small ones) to cross from the brood box into the honey super, but not the queen bee (the big one). The intent of the queen excluder is to limit the queen's access to the honey supers. If the queen lays her eggs in the honey supers and a brood of young bees develops, it is difficult if not impossible to keep their waste out of the clean honey.
The deluxe queen excluder pictured above is made of fir. Its dimensions are 41 X 51 cm and it's a much more eco-friendly alternative than the plastic ones that many beekeepers use.