A 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 8 inches (20 cm) long, and make about 16 to 18 pats of butter. Most are stainless steel and aluminum; Most are dishwasher safe.
To use, start with cold butter — not softened butter. Place it on top the butter, line it all 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.