A butter pat slicer (aka butter slicer / butter cutter) is a long rectangular frame, open in the middle, with thin wires running crosswise (as opposed to lengthwise) through it.
It is designed to cut a standard-sized 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 made of stainless steel and aluminum. Some more recent ones may have the frame made of plastic, with the cutting wires still metal. Some will have markings indicating right-side up.
Most modern ones are labelled dishwasher safe.
To use, start with cold, hard butter — not softened butter. If you cannot buy standard-sized sticks of butter where you are, take a pound (500 g) of butter, and quarter it lengthwise before starting. Remove any wrapping on the butter. Place the slicer on top of the butter, right-side up, line it up, and then press it down all the way through the butter.
Store the butter pats covered in the refrigerator, or in ice water.
You can also use a butter cutter to slice bananas, or to mark up rolls of cookie dough for cutting.