An exposure is any group you've called and laid face-up in front of you on the table: a pung, a kong, a quint, whatever it happens to be. The second it's exposed, it's locked in. No swapping it out, and everyone at the table can now see exactly what you're holding.
There's a real trade-off here. Exposing early speeds your hand along, but it also hands the whole table free information. A sharp opponent can often guess what you're building well before you're actually close. Which is exactly why some players will pass up a call they could easily make, just to stay concealed a little longer.
Nobody's got a universal answer for which way is right. It depends on the hand, how close everyone else looks, and honestly, how much you enjoy keeping people guessing.
New to the table? Start with mahjong terms explained or our step-by-step guide to American mahjong.