A joker is one of the eight extra American tiles. It can finish a pung, a kong, or a quint of identical tiles.
Each natural tile exists in four copies, so a quint almost always needs a joker. Without one, that group cannot be made.
It cannot finish a pair, a single, or mixed tiles. Flowers are not jokers. The white dragon is not a joker.
It never leaves the rack in the pass
Jokers never travel in the Charleston, a blind pass, or the courtesy exchange. Riichi, Hong Kong, and Sichuan have no jokers. If someone says joker, the table is probably American.
See the joker lesson. Related: pair, quint, Charleston.
A joker cannot finish a pair.