A Concealed hand in Mahjong Island is a hand on the Island Card marked C. You build it from tiles you draw yourself.
You win by completing one hand on the Island Card: 14 tiles that match one line.
What does the C change?
The C changes how you may collect tiles. On a hand without the C, you can use Take Tile when another player discards. Take Tile means picking up that discard. You may do it to make an Open Group of 3 to 6 identical tiles that your hand still allows.
A Concealed hand gives up that choice. You build its Groups from your own draws; only the winning tile can come from a discard.
A Pair works the same way: two identical tiles, like two Red Dragons.
How do you win a Concealed hand?
You win a Concealed hand the same two ways as any hand. You can draw the last tile yourself, called a Self-Drawn Win. Or you can take the one discard that completes your hand.
To see how every line on the card is read, visit how the Island Card works. For the full turn order, see the Mahjong Island rules guide.