A mushroom cutter is a device used for slicing a mushroom into uniform slices all at once.
How they work will vary, but most will have tines or blades that the mushroom is forced through in order to achieve the cutting.
You raise the top handle, exposing the space on top of the blades in the bottom part.
You put the food on the blades, and partially close the top handle, which puts a pusher over top the mushroom.
Then you press down on the handle. This forces the tines through the mushroom, slicing it.
Mushroom slicers can be also used to slice radishes, kiwi fruit, hard-boiled eggs, and strawberries.
Some advise that you can use egg slicers to slice mushrooms with. You absolutely can, if you are sure of the quality of your egg slicer. It needs to be a very heavy-duty one, with proper blades, rather than just light wire for blades, otherwise the wires may just pop out, ruining the egg slicer for anything. That being said, not every mushroom cutter is very heavy duty, either.