The Heart Medal increases the chance of finding recovery hearts after defeating enemies, cutting down grass, etc.
If you often find yourself taking a beating, you could consider keeping one or two of these in your item pouch.
That said, unless you’re also carrying the Cursed Medal (which prevents you opening your pouch), it’s more reliable to have Bottles with potions inside instead.
Where to Find
There are two Heart Medals in the game.
Inside Goddess Chest 20, accessible once you unlock Lanayru Desert.
Inside Goddess Chest 21, which requires the Water Dragon’s Scale to reach.