A splatter screen is a screen consisting of a solid rim with mesh stretched between the sides.
It acts as a cover to a pot or frying pan to stop fat, grease and liquid from splattering around the cooking surface, whether stovetop or microwave, while still allowing steam to escape and preventing food from getting soggy.
The screens come in varying widths. They sometimes sold in packs of varying sizes.
There are plastic ones for microwaves. Not all metal ones are dishwasher safe. You can also get ones that are metal with a non-stick coating.
Some have feet on the edges, so that you can set them on the counter without the mesh touching and dirtying the counter that you were trying to keep clean and free of splatters in the first place.
Typically they are round, though they can also be found in other shapes. And, they typically have handles for easy lifting off from a hot pot or pan while keeping your hands away from the heat. Some may have handles that fold in for more compact storage.
Some people also use their splatter screens as improvised strainers or sifters.