Strop

A strop is a piece of leather, or a hard, leather-like substitute, charged with a fine polishing compound (green chromium oxide is the most common) and used to put a final, mirror-polished edge on a chisel or plane iron after sharpening on stones. Drawn across the strop trailing-edge-first (never leading-edge-first, which would just cut into the leather), it removes the microscopic wire edge sharpening stones leave behind and refines the edge to its sharpest practical state.

Stropping isn't a replacement for real sharpening on stones -- it can't fix a dull or damaged edge, only polish one that's already been properly ground and honed. Used as a quick touch-up between full sharpening sessions, though, a few strokes on a strop can meaningfully extend how long an edge stays working-sharp.