Oh Hit Points you lovely aged and timeless mechanic for measuring a character's capacity to endure harm. They're luck, divine favor, toughness,vitality, pounds of flesh, and pints of blood all rolled into one that an awful lot of people really have a tough time coming to grips with.
At their core a character with more HP than another character is more important than a character with less HP (on the battle field and in meat-grinder dungeons). More of the tale is going to be about the guy with more HP, becasue they will be there longer...most of the time. HP only get weird when folks start thinking of abstract values in terms of absolutes. Some folks see a blow that causrd 8 HP of damage as the same 8 HP of damage regardless of the target being struck. This is a limited view of HP and how damage is dished out in the game that serves to confuse.
In the simplest form HP are how much of a beating one can stand until they can stand no more. Some folks really hate this, they feel every blow (or almost every blow) should reflect some sort of wear and tear; other folks believe HP represent a threshold from "doing fine" to "done fighting".
The value to HP is they can offer a relative balance of estimated staying power, harm, and recovery within game-able context. Sure it seems odd one can heal luck and divine favor but really folks, how many different score do yuo want to track in your RPG? Stick with one score for dead and not dead.
Hit points are abstract, arrows in a quiver aren't; an arrow represents an arrow not an abstract ability to inflict harm over distance. What is abstracted is the degree of harm the arrow causes following the discrete use of the arrow by an archer, not the number of arrows used. How many arrows does it take to kill a warrior with 90 HP....the last arrow. The rest of the arrows skimmed him, didn superficial damage, slapped armor hard enough to cause a minor degree of trauma but htey dsisn't stick leaving the target looking like a pin cushion. Unless of course HP aren't abstract in the game you are playing and then things get more complicated.
I really don't care for the now you are fine, now you are done nature of HP and I don't care for yo-yo fights caused by easy healing magics. I favor HP as a buffer between "doing okay" and "now you are in trouble" but it isn't an exclusive notion it's one that varies with the needs of the campaign and the rules of the game begin used. No one gets arms lopped off fighting at 80 HP (unless special powers and techniques are being applied) until HP are 0 or less becasue HP are a measure of multiple factors and characters don't run out of luck until they run out of that luck, divine favor, toughness, vitality, pounds of flesh, and pints of blood.