Definition
A concealed hand is one the card requires to be completed entirely in your own rack, with no calling except for the final winning tile.
Concealed hands pay more because they are harder. Calling any set on a concealed line makes the hand dead.
Read the full guide: How to Read the Mahjong Card: The Notation, Decoded.
See it on real tiles: the coach reads any hand and names its direction, free, at the free read. Members get the coach on call for their own games.
More terms: Pung, Kong, Quint, Pair, Charleston, Blind pass, Courtesy pass, Joker.