Heart containers represent your maximum health in The Binding of Isaac: Rebirth (TBOIR)
A hear container can be:
- Full (solid red)
If all heart containers become empty, Isaac will die
You cannot pick up red hearts if all your heart containers are full, but you can come back to the room later and pick it up after you take damage
Most enemies and other damage sources will take away either half a heart or one heart.
Different characters start with a different number of heart containers, for example Isaac starts with three heart containers. However, various items can either lower or raise your number of containers.