Cloak is on of six standard Action Bar actions.  It is available on all Romulan and most Klingon ships.

The ship must have at least 1 Active Shield to perform a Cloak Action.  Disable all of the ship's remaining Shields and place a Cloak Token (green side) on top of its ship base. During the Combat Phase, the ship gains +4 defense dice.  If the ship fires, flip the Cloak Token over to its red side; the ship is still considered Cloaked for the remainder of its round.

During the End Phase, if the Cloak Token is red, remove it.  If the Cloak Token is green, the owner may keep its Shields disabled to retain the Cloak Token; move the token off the ship base to signify that it's been Cloaked for more than one round.