Hit points are a measure of a character's health or an object's integrity. Damage decreases current hit points, and lost hit points return with healing or natural recovery.
Hit points cannot normally exceed the character's maximum hit points (the main exception being temporary hit points).
Hit points per level[]
Each class has a set amount of hit points for each of its levels a character has. The specific amount each class level adds is specific to that class. See hit die for more details.
Consitution[]
Any increase in a character's constitution bonus also retroactively increases the maximum hit points. If this is not a permanent effect, the additional hit points will also disappear when the bonus is lost. Similarly, a drop in constitution bonus will cause a loss in hit points (temporarily or otherwise).