Receive 10x Weak Heart to permanently boost your health. 

By default you have 20 health (10 hearts), by consuming 10 Weak Hearts you increase this to 40 health (20 hearts).
Up to 10 Weak Hearts can be used before you reach a hard cap and can no longer boost your health using Weak hearts.

To continue boosting your health beyond 40, you must use Heart Crystals. Crafting Heart Crystals requires Weak hearts.