Nail Set

A nail set is a small steel punch with a slightly concave or flat tip, placed on a nail head that's already been driven nearly flush, then struck with a hammer to sink the head a millimeter or two below the wood's surface. The resulting small depression gets filled with wood filler or putty, hiding the fastener entirely rather than leaving a proud metal head visible (and prone to catching a stray hammer blow that dents the surrounding wood).

Set sizes are matched to nail head diameter -- too small a tip can slip off an angle and mar the surrounding wood, too large won't seat cleanly into the head.