Hit points (sometimes called health points and often abbreviated HP) are a numerical measurement of a unit's (or city's) physical condition in the Civilization games. Military units become less effective in combat as they lose hit points, and a unit reduced to 0 HP dies.
The earlier Civilization games do not always delineate the difference between hit points and Strength clearly. In Civilization II, a unit that wins a round of combat inflicts damage equal to its Firepower, which is subtracted from the losing unit's hit points.
In Civilization III, a unit that is drafted or acquired from Barbarians starts as a "conscript" with 2 HP; a unit produced by normal means starts as a "regular" with 3 HP, gaining +1 HP when it becomes a "veteran" and +2 HP when it becomes an "elite" (for a total of 4 and 5 HP, respectively). Adding units to an Army pools their hit points, allowing an Army to have three times as many hit points as a single unit (or four times as many for a player who owns The Pentagon).
In Civilization V, every unit has 10 HP, and every successful attack inflicts at least 1 HP in damage. This is increased to 100 HP in Gods & Kings, making it much more difficult for primitive units to defeat advanced ones with horde tactics. The only units that do not have hit points are the Caravans and Cargo Ships from Brave New World, which can be plundered by an enemy military unit that moves into their tile.
Civilization VI largely follows the trend from Gods & Kings, giving all military and religious units 100 HP (though prior to the Fall 2017 Update, a military unit would instantly kill an enemy's religious unit if they occupied the same tile). Support and civilian units (including 
Great People) cannot lose hit points in combat, and most can be killed, captured, or plundered by an enemy military unit that moves into their tile.