"Unique" is a keyword belonging to cards that represent singular instantiations. These particular instantiations cannot simultaneously be singular and multiple: thus the rule for Unique is that only one Unique of the same name may occupy the playing field at a given time. If another Unique of the same name is played, the first is destroyed, and the second replaces it.
Whether a card is Unique or not can be seen to the left of the Faction Icon on the card.
Similar to the Legendary restriction from Magic: The Gathering.