A special ability that allows a unit to become invisible on the field. Its similar to Stealth. You might be lucky to have some visual cue that lets you know an enemy cloaked unit is around, like a shimmering shadow, but more than likely its surreptitiously spying on you without your knowledge. Worse of all, if it can fight cloaked, it will be merrily killing your helpless units: the usual RTS convention is that units can only target things that they can see or know of. Anything that's cloaked can't be shot at and acts with impunity. Cloaked units are usually countered by Spotters, who can detect invisible units. Some specialised units can cloak any friendly units near them within an area-effect cloaking field.
If a card has Cloak it is kept face-down when in play and cannot be targeted specifically by Spells or abilities, but is still affected by Spells and abilities that do not target the card directly. For example, "Destroy target Creature/Guard" cannot affect a card with Cloak, but "Destroy all Creatures/Guards currently in play" can affect a Cloaked card. Also, If another player does not know anything about a Cloaked card, you don't have to tell them, except for if it's in play or not. Cloaked cards are always 'upright' like a Withdrawn card, but face-down. This is because if a Cloaked card affects anything in any way, even itself, through passive, active or innate abilities, being used to Create a Specific Spell, et cetera; it must be flipped face-up so other players can confirm that the card is really what it needs to be for such a thing to occur. Any cards with Cloak regain it and flip over face-down again during your next Maintenance Phase.