Seal Cards are unusuable until you submit a word of the indicated length, which breaks the seal and turns them into a normal card. Seal Cards may have a number as low as 2, or as high as 9.

NOTE: If you have multiple Seal Cards on the board, you may be able to break mulitple seals by submitting a single word. For example, in the image below, if you made a 6-letter word it would break both seals.