The Tsuba is the hand-guard of the japanese sword also called Katana.
The Tsuba changed of utility over time from hand-guard to sword ornament. Indeed, during the Muromachi (1333-1573) and Momoyama period (1573-1603) they were more functional, made in solid metals and of very simple design, then came the Edo period (1603-1868) and his era of peace who made the Tsuba an ornament, which made their designs becoming increasingly aestheticized, but less practical for combat.
Tsuba were made by family specialized in its production and was often transmitted from generation to generation. Many families had their samurai crest or "mon" inlaid or engraved on them.