A concealed hand is a hand that does not allow for exposures. In other words, you cannot call a discard and expose tiles. If you do and later call Mah Jongg, your hand will be declared dead (even if you have all the tiles matching that hand).
However, you can call a discard if this is the last tile you need for Mah Jongg.
Concealed hands are indicated by the letter "c" near the number of points for the hand on the card.