An onion slicing guide (aka onion or tomato holder) is a tool to help you cut paper thin slices of things such as onion and tomato.
One design is like short-handled slatted tongs. You peel the onion, and then grip it between the slatted tong heads. The tong heads have long tines in them. You cut down through the tines to make thin, even, straight slices. These are made of plastic or aluminum and have a mechanism on the handle to lock the tongs together to make them less unwieldy to store.
Another type of guide resembles an angel food cake cutter, only the tines are shorter. You peel an onion, cut it in half, place the onion cut-side down on the cutting board, insert the guide, and then slice between the tines. This can also be used to hold something while you are cutting it, to keep your fingers safely out of the way, and as a slicer for cakes that aren’t as tall as angel food cakes.