A dead hand is a hand that can no longer win — usually because it's missing a tile that's already been discarded and passed, made impossible by an exposure, or simply doesn't match any hand on the current card anymore. Once a hand is dead, that player is out of contention for the rest of the round; they keep playing (discarding, mostly) but can't call mahjong.
Hands go dead more often than beginners expect, usually from chasing a hand too long after the tiles needed for it have clearly dried up. The fix isn't complicated: keep half an eye on what's been discarded, and be honest with yourself earlier than feels natural about whether a hand is still alive.
A dead hand isn't a disaster — it happens to good players constantly — but recognizing it early saves you from playing three more rounds toward a hand you were never going to complete.
Keep going with what an exposure is or our step-by-step guide to American mahjong.