Each character has a varying number of HP, just as monsters do.
These hit points represent how much damage (actual or potential) the
character can withstand before being killed. A certain amount of these hit
points represent the actual physical punishment which can be sustained.
The remainder, a significant portion of hit points at higher levels, stands
for skill, luck, and/or magical factors. A typical man-at-arms can take
about 5 hit points of damage before being Killed.
Let us suppose that a Lord (F10) has 55 HP,
plus a bonus of 30 hit points for his constitution, for a total of 85 hit points.
This IS the equivalent of about 18 HD for creatures, about what it would take to kill four huge warhorses. <>
It is ridiculous to assume that even a fantastic fighter can take that much punishment.
The some holds true to a lesser extent for clerics, thieves, and the other classes.
Thus, the majority of hit paints are symbolic of combat skill, luck (bestowed by supernatural powers), and magical forces.

Hit points are determined by hit dice.
At 1st level a character has but one HD (exception: rangers and monks begin with two dice each).
At each successive level another hit die is gained, i.e. the die is rolled to determine how many additional hit points the character gets.
Hit points can be magically restored by healing potions, cure wounds spells, rings of regeneration, or even by wish spells.
However, a character's hit points can never exceed the total initially scored by hit dice, constitution bonus (or penalty), and magical devices.
For example, if a character has 26 HP at the beginning of an adventure, he or she cannot drink a potion or be enchanted to above that number, 26 in this case.

As an example, let us assume that the character with 26 HP
mentioned above is engaged in on adventure. Early in the course of
exploring the dungeon, he or she falls into a 10' deep pit taking one
six-sided die (1d6) of damoge - 4 hit points drops to 22 HP.
Next, he or she takes 15 points of damage in combat,
so the character drops to 7 HP.
A cleric in the party uses a cure serious wounds spell on the character,
and this restores 10
(for example, depending on the die roll)
of his or her lost HP,
so the character has a total of 17.
Later activities reduce the character to 3 HP,
but the party uses a wish spell to restore all members to full HP, so at that time the character goes up to 26 once more.

REST also restores HP, for it gives the body a chance to heal itself and
regain the stamina or force which adds the skill, luck, and magical HP.

Your character's class will determine which sort of die you will roll to
determine hit points. In some campaigns the referee will keep this total
secret, informing players only that they feel "strong", "fatigued" or "very
weak", thus indicating waning HP. In other campaigns the DM will have players record their character's hit points and keep track of all changes.
Both methods are acceptable, and it is up to your DM as to which will be used in the campaign you participate in.