Player Character hit points at level one are equal to the character's Constitution score (actual score, not the modifier). At level two and beyond the character receives additional hit points based on their chosen class. Hit points represent not only flesh and blood, but the effects of exhaustion, poison, disease, extreme temperatures, and other status changing events.
Hit points can be regained up to the original maximum in several ways. These include, but are not limited to; eating food, rest, medical attention, healing potions, and magical effects.
When Hit Points are reduced to zero, the character is killed or rolls on the Death and Dismemberment table.
Example: Louise the Lesser is a a first level fighter with a Constitution score of 16. This means Louise's hit points are also 16. At second level, Louise rolls a d8 (because they are a fighter) and gets a 5. This is added to 16, making Louise's hit points at 2nd level 21. In battle with Talrog the Terrible, Louise suffers terrible injuries, culminating on a roll on the Death and Dismemberment table; Louise is lucky (perhaps earning a new nickname?) and suffers a hideous scar on their face.
Non Player Characters
Non Player Characters use hit points a little differently
NPCs and monsters have Hit Dice. Each HD is the equivalent to d8 hit points. Unlike player characters, they do not add their Constitution score to their hit point total. Also unlike player characters, monsters are killed or unconscious when their hit points reach zero; they do not roll on the Death and Dismemberment Table (unless the GM wants to).
Example: Talrog the Terrible has 8 hit dice. The GM rolls them and comes up with 52 hit points. Talrog manages to hurt Louise the Lesser in battle, but is ultimately defeated and killed when his hit points reach zero.