A queen excluder is a sheet of material with holes in it. The holes allow workers, who are a little smaller, to get through, and the queen cannot. The queen excluder hole size is crucial to its functionality.
A queen excluder is a sheet of material or a mesh with holes in it. The holes are sufficiently large that workers can just pass through. The queen is larger, and she is unable to pass through these holes. This is similar to the idea of bee space.
A queen excluder can be used as a management tool to confine the queen to one or two brood boxes. This limits colony size and forces bees to store honey in the boxes above the excluder. It also reduces the risk of getting brood in your supers. This makes it easier to extract clean honey.
I have over the years done a lot of beekeeping consulting work in developing countries. Africa and parts of South America are especially difficult regions to source equipment in. Things are either unavailable or expensive.
Bees are complex animals, and the environment is a very complex thing as well. Queen excluders are a tool – as with any tool, there is a right time and a wrong time to use them. Experience shows us when to use tools. That is what makes us human.