2-shanten: needs 2 more useful draws

33

44

77

After discarding 7c: still 2-shanten (isolated tile kept options)

33

44

Shanten measures how many tiles away you are from tenpai (ready to win). A hand at 2-shanten needs 2 useful draws to reach tenpai. When discarding, always pick the tile that reduces shanten the most — or at least keeps the most future options open.