In Lost Ark, there are different kinds of collectibles that you can search for to receive rewards. One of these collectibles is called Giant's Hearts. They look like gems and are described as the heart of a giant who fell during the Hollow War. These collectibles can be acquired through various means, as outlined in the Collectibles list.
The Tenth Giant's Heart is one of the easiest you can get. In order to acquire it, you must have access to The Tower event and have an Item Level of at least 480.
When you arrive at The Tower, you will need to reach Floor 35. This floor requires you to defeat a boss named the Elemental Knight of Darkness to proceed. Using your Awakening ability or pelting him with melee attacks will eventually take him down, defeating the floor.
One of the rewards you get for completing this floor is the Tenth Giant's Heart. You don't need to select it in your inventory or do anything to bind it to your character to receive it.
Want to see more about classes in Lost Ark? Check out Best Classes to choose in Lost Ark on Pro Game Guides.