A card's attributes (if it has any) will be listed just above the card's energy. An attribute could be any number of things, but by far the most common is Invincibility to instant killing attacks.
Attribute List[]
Invincible to instant killing attacks[]
This attribute is present on many cards, and prevents an enemy from using a "kills any card" attack on the card. Note: This does NOT protect from attacks labeled "kills any card regardless of invincibility to instant killing attacks."