Heart Containers are items found in The Legend of Zelda series. They are heart-shaped crystals, imbued with vitality, that increase Link's maximum HP by one heart. Since The Legend of Zelda series uses an equipment based progression system rather than a level based one typically found in RPGs, Heart Containers essentially allow Link to "Level Up" his health.
Full Heart Containers are typically dropped by defeated bosses at the end of dungeons. These will automatically give Link a boost to his maximum HP if he picks them up. Additionally, Link can find smaller pieces of heart hidden around the game world or awarded for completing side-quests that combine once four to five are collected into a full Heart Container. Heart Containers have appeared in nearly every game in the series excluding Zelda II: The Adventure of Link for the NES which used a more traditional leveling system.