An exposure is any set of tiles you've called and laid face-up on the table in front of you — a pung, a kong, or a quint you've picked up mid-game rather than kept in your hand. Once tiles are exposed, they're locked: no swapping them out, and every other player at the table can see exactly what you've got.
Exposing early can speed up a hand, but it also hands the table information. A sharp opponent watching your exposures can often guess what you're building before you're close to mahjong — which is exactly why some players hold a tile they could call and stay concealed instead, trading speed for secrecy.
There's no universally right answer here. It depends on the hand you're chasing, how far along everyone else looks, and honestly, how much you enjoy making people guess.
New to the table? Start with mahjong terms explained or our step-by-step guide to American mahjong.