Giant’s Hearts are a collectable item registered in your Collectables window (Alt + L).
There are currently only 15 Giant’s Hearts in total and although the first few can be easy to track down, some of these collectables will take a long time to obtain.
The collection rewards include skill points, masterpieces, skill runes and stat potions.
Once you have collected a Giant’s Heart, you must go to Wisdom Isle and speak to “Minerva” to register the Giant’s Heart in your Collectables window. You can also claim the Collection Rewards by speaking to her.