Cards have four different stats: Cost, HP, ATK, and SPD.
Cost is the energy requirement to place a card on the battlefield.
If a card's HP is reduced to zero, it will no longer be able to act during the current turn. During the next turn, the card's HP will fully recover and it will be placed back on the battlefield.
ATK determines the amount of damage a card deals when attacking.
SPD affects how quickly a card can act during battle. The higher the value, the sooner it gets to take action.