HP, or health points or simply health, is a number that dictates how many hits a card can take before it is destroyed. The higher the number, the longer a card will last in battle.
HP is a part of a card's basic info, and it is listed directly under a card's basic attack.
HP, like POW points, is measured on a scale of thousands. When making cards, this MUST be taken into thought. If you make a card with a number such as 50 for its HP, even the weakest attack will be able to take it out!
It is often difficult to keep track of how much HP is gained and lost in battle, so it is recommended that players use calculators to keep track of the status of their cards' HP. This way, keeping track of HP is much easier, and there will be no confusion.