A truffle shaver is a small hand tool designed to make small thin shavings from truffles (the mushroom ones, not the chocolate ones.)
Most are shaped like a small paddle, about 15 cm (6 inches) long, with a single flat plane straight blade in the centre.
The blades are always stainless steel; the paddles can be stainless steel as well, or wood. Some truffle shavers have a defined handle, some are all just paddle.
Most let you adjust the blade for other thicknesses.
A truffle shaver can also be used for cheese, carpaccio and chocolate shavings.
Substitutes
Mandoline, or a sharp knife and a steady hand.