A queen excluder is a grid of wire or slotted metal/plastic placed between the brood box and the honey supers. The gaps are wide enough for worker bees to pass through freely, but too narrow for the larger-bodied queen (and drones) to fit through. That confines egg-laying to the brood box below, so the frames in the honey supers stay free of brood and pollen — pure honey that is easy to extract.
Some beekeepers nickname the queen excluder a "honey excluder" because a poorly made or dirty excluder can measurably slow how fast workers carry nectar upward — which is exactly why wire spacing and super timing both matter for a productive honey harvest.
A 3D cutaway hive shows the queen physically stopped at the wire grid of a queen excluder while worker bees stream freely up into the honey supers above — and what happens to brood risk and honey flow when you change the setup.
Excluder wire spacing is sized between a worker's thorax and a queen's — wide enough for workers, too narrow for the queen — so brood stays out of the honey supers while nectar still moves upward.
Toggle the excluder, tighten or widen the wire spacing, add more supers, or switch on checkerboarding to see how each choice changes queen location, worker traffic, brood risk and honey fill rate.
Some beekeepers call it a "honey excluder" — a poorly finished or too-tight excluder can noticeably slow how fast bees carry nectar into the supers, which is why spacing and timing both matter.