HP stands for hit points. When a party member gets 0 hp, the character is knocked out. Once every party member's HP hits zero, the battle is over, and the opposite side wins. The player can view their side's HP, but cannot view their opponent's. Wakey tails can revive any knocked out party member.
Gummy Frogs can restore a varying amount of HP based on the color. Green gummy frogs are most common, and revive 150 HP.