The Lock special when activated locks the next letter played on that cell. The letter will not be cleared when a word is spelled using that letter. Certain levels will appear with an activated lock cell already in place on the board. Locked cells clear at the end of the level.
A Lock special first appears with the lock icon. When you activate that special by spelling a word over it, the lock icon is replaced by an X icon. When you then drop a letter over that cell, the background will turn red, and that letter will be locked for the remainder of the level.
Hint: Playing a vowel on this cell can make it easier to spell words as you progress.
Tip: Try and get three or four locks in a row, spelling a word. Doing this will continuously score that word when the string clears the board!