A Joker in Mahjong Island is a wild tile. It stands in for a tile in a Set, Quad, Quint, or Sextet. It can never be a Single or part of a Pair.
You win by completing one hand on the Island Card, Mahjong Island's own card of hands. A hand is 14 tiles that match one line on the card. Each line is written as groups of identical tiles, like a Set (3 matching tiles) or a Quad (4 matching tiles).
How does a Joker fill a gap?
A Joker takes the place of one missing tile in a group. Say you hold two 5 Crak tiles and need a third for a Set. A Joker can be that third tile. Now your Set is done.
A Set built this way is sometimes called a pung in traditional mahjong. Jokers are not only a Mahjong Island idea. American mahjong sets include joker tiles as well.
Where can a Joker not go?
A Joker never fills a Single or a Pair. A Pair is 2 identical tiles, and both must be real. Jokers are also never passed in the Charleston, the tile passing before play begins.
If a Joker is discarded, it is dead. Nobody can take it. For the full rules of a turn, read Mahjong Island Rules: How to Win a Hand.
Do Jokers score points?
Yes. A Joker scores the Chips, or points, of the tile it stands for. Each game deals 8 Jokers, each a different kind, from 16 core kinds.
Each kind also adds its own Mult, a multiplier on your score. The Palm Joker adds +2 Mult, for example. See How Mahjong Island Scoring Works for the full picture.