CHARACTER HIT POINTS (PHB: p.34)
Each character has a varying number of hit points, 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 10th level fighter has 55 hit points, plus a bonus of 30 hit points for his constitution, for a total of 85 hit points. This is the equivalent of about 18 hit dice 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 character has but one hit die (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 hit points 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 hit points 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 damage - 4 hit points of damage - so the character drops to 22 hit points. Next, he or she takes 15 points of damage in combat, so the character drops to 7 hit points. A cleric of the party uses a cure serious wounds spell on the character, and this restores 10 (for example, depending upon the die roll) of his or her lost hit points, so the character has a total of 17. Later activities reduce the character to 3 hit points, but the party uses a wish spell to restore all members to full hit points, so at that time the character goes up to 26 once more.
Rest also restores hit points, for it gives the body a chance to heal itself and regain the stamina or force which adds the skill, luck, and magical hit points.
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 hit points. In other campaigns the Dungeon Master 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.