Shutter Cards swap every turn between open and closed. When they are open, just use them as you would any normal card. But when they are closed, they are unusuable; you'll have to make a word using other cards, in order to get them to open.

In the picture below, you'll see that on the left, the arrows highlight which cards are open and which are closed. On the right, after a word is played, those states switch — previously closed cards open, and previously playable cards close.