Hit Points (or HP) is a number representing a creature's life force, or an object's structural integrity. It indicates how much damage you or party members can take before being Downed, and how much damage creatures and items can take before being killed or destroyed.
Taking damage affects how injured your character appears, with more cuts and bruises appearing as HP percentage drops.
The maximum number of Hit Points possessed by a player character is primarily determined by their Class:
- Barbarians start with 12 HP and gain +7 with every level up.
- Fighters, Paladins and Rangers start with 10 HP and gain +6 with every level up.
- Bards, Clerics, Druids, Monks, Rogues and Warlocks start with 8 HP and gain +5 with every level up.
- Sorcerers and Wizards start with 6 HP and gain +4 with every level up.
Characters also receive additional Hit Points equal to their 
Constitution modifier for each character level. For example a level 3 Fighter with a Constitution Score of 14 and thus a modifier of +2 would have a Hit Point maximum of 10 + 6 + 6 + (+2 * 3) = 28. This also applies for negative Constitution modifiers, which reduce the maximum HP possessed by a character. The Hit Point maximum is adjusted up or down accordingly if a character's Constitution modifier changes.
Characters cannot have fewer than zero Hit Points, or more than their Hit Point maximum, except for Temporary Hit Points.
Temporary Hit Points
Temporary Hit Points are additional Hit Points that are lost before your base Hit Points.
You can only have one set of Temporary Hit Points at a time, and healing can't restore them. A second source of Temporary Hit Points will overwrite the first if the second gives more Temporary Hit Points than the first source's initial value, regardless of how much of that initial value remains.[Needs Verification]
Example: Level 1 Heroism applies 5 Temporary Hit Points when cast, and again each turn until it expires; and Level 1 False Life gives a one-time 7 Temporary Hit Points. If you have Heroism on you, and then you use False Life, you will have 7 Temporary Hit Points instead of 5. If you take 4 damage, you will have 4/7 Temporary Hit Points, but Heroism's 5 will still not overwrite that, neither through its per-turn applications, nor if you receive another casting of Heroism. However, if you lose that remaining 4/7 Temporary Hit Points, the next per-turn application of Heroism (if it's still active) will give you 5/5 Temporary Hit Points.
Taking damage to your Temporary Hit Points will still cause a Concentration check (when Concentrating), even if the damage doesn't overflow into your base Hit Points.
Temporary Hit Points do not remove Downed and disappear after a Long Rest.