What is tenpai, and why does it matter?

Tenpai (聽牌) means your hand is one tile away from winning. You are "waiting" for a specific tile or set of tiles to complete your hand. Recognizing when you are in tenpai is crucial because it determines which tiles are safe to discard and which to hold. In some rulesets, being in tenpai at the end of an exhausted wall hand earns a small payment from players who are not in tenpai. Skilled players also try to identify when opponents are in tenpai to avoid discarding their winning tile.