A butter pat slicer (aka butter slicer) is a long rectangle frame, open in the middle, with thin wires running crosswise (as opposed to lengthwise) through it.
It is designed to cut a stick of butter into even pats of butter.
It will be about 20 cm (8 inches) long, and make about 16 to 18 pats of butter. Most are stainless steel and aluminum. Most are labelled dishwasher safe.
To use, start with cold butter — not softened butter. Place the slicer on top the butter, line it up, and press it down through the butter.
You can also use it to slice bananas with, or to mark up rolls of cookie dough for cutting.