Health is one of the two main stats in Inscryption. When being attacked, a Card's health drops equal to the Power of its attacker. When Health reaches 0, a creature dies and is removed from the match, going back to your deck if you were its original owner. When killed, creatures give you Bones.
Health will also drop to 0 instantly if the card is sacrificed, unless it has the Many Lives sigil.
Some Sigils also interact with death, such has the Unkillable (which adds the card back to your hand when it dies) and the Bone King (which adds 4 bones to your Bone pile, instead of 1).