Broken is a term commonly used in collectible games to refer to something which is overly powerful.
The Power Nine, for example, are considered to be so extremely broken that just about every deck which can include them should do so. Taking an otherwise average or underpowered card and utilizing it in an extremely powerful way is called "breaking" the card.
Despite the damaging connotation of the word, it is considered very good for something to be "broken" in gaming.