Take a discard in Mahjong Island when it wins the hand or finishes a Group your best hand truly needs. Otherwise, draw and keep your hand hidden.
A discard is a tile another player has just thrown away. Taking it (Take Tile) means you claim that tile instead of drawing a new one (Draw Tile). A Group is a set of identical tiles your hand needs, such as a Set: three matching tiles.
Here is one example. Your best hand needs a Set, and you hold a Pair, two of those matching tiles. Another player discards the third copy. Now you choose: take it and finish the Set, or let it go and draw.
What does taking a discard cost?
Taking a discard finishes a Group at once, but that Group goes face up. Everyone at the table can now see which hand you are chasing. Your hand also can no longer score Fully Concealed.
Fully Concealed is worth +3 Mult when you win without taking a discard into a Group. Mult is the multiplier on your score, so losing it shrinks a win. You can read more in the guide to Chips × Mult scoring.
When should you win on a discard?
Always, if it completes your hand. You win with 14 tiles that match one line on the Island Card. When you are one tile away, any player's discard of that tile wins.
The habit to take to your next hand: one tile away? Any player's discard can win. For more habits like it, read Mahjong Island strategy for beginners.