A queen excluder is a tool used in beekeeping to separate a beehive into two parts; a lower portion (the brood supers), where the queen is free to roam and lay eggs, and an upper portion (the honey supers), which the queen can't get to and hence can't lay eggs in. The idea is that you really don't want brood mixed in with your honey. When they're not used, it's usually a very complicated and difficult process to separate the honey out.
It usually consists of a thin sheet with holes in it, placed between the two super types; worker bees can fit through the holes, the queen bee can't. A common alternative is placing a grid of appropriately-spaced metal bars/wires in parallel, placed in the same position. Normally made out of plastic or metal, beekeepers sometimes create their own by simply drilling appropriately-sized holes in a piece of wood.
The term is also sometimes used to refer to parallel bars placed inside the hive entrance to stop the queen from leaving the hive. Those are usually only left there for a short time, until the queen starts laying eggs (at which point most types of bees will not leave the hive - however, some will, so don't go removing it on the advice of this writeup and do something silly like unleashing killer bees on your neighbourhood!).