An onion slicing guide (aka onion holder) is a tool to help you cut paper thin slices of things such as onion and tomato.
One 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.
Another more elaborate type of guide is more like tongs. You peel the onion, and then grip it between the tong heads. The tong heads have long tines in them, and cut down through the tines to make thin, even, straight slices. Most are made out of aluminum and have a mechanism on the handle to lock the tongs together to make them less unwieldy to store.