I can answer this question. In fact, it’s been answered by people who are far more eloquent and educated than I; but because I like to write, I’ll offer my take on it. In a moment. Not just yet. Because there are underlying issues that should be addressed first.
The question, “What are hit points?” often comes with other questions or critiques. Things like, “hit points are abstract concepts,” or, “they can’t be measured in real life,” or, “there’s no way a character would know what hit points are or how many a given creature has.” Really, each of these follows from attempting to answer a different question: how do the rules of the game relate to the real world?
Please understand we’re not attempting to simulate the real world. This is not desirable from a game design point of view because the real world is simply too complex for us to manage without the aid of equally complex systems, which would probably require a computer to employ, and at that point… well, we’re not at that point. Maybe, some day, but not today. So simulation is not the purpose behind the question of how rules relate or correspond to the real world. The purpose is providing information to the players.
If I can convey to my players that an 11 Dexterity means a certain thing ~ like, for example, how fast you can run in feet/second ~ then I’ve created a point of common ground; there’s a shared understanding between us; and this speeds up play by eliminating the need to explain again and again how different characters or monsters are faster, slower, quicker, more agile, etc. as they relate to each other.
It also has the benefit of helping the player translate non-numerical data about the world into numerical data (or approximations) which aids his ability to make informed decisions about the game. But more on that in a moment.
We’re not trying to simulate the real world. We’re creating a game. Which means we start by defining our terms and assigning value to them. Then we can associate those values with real-world analogues. For example, Strength is a fairly common ability or attribute in most RPGs, so let’s start there. On a scale of 3 to 18 (3d6), we can say that a Strength of 10 or 11 is average. What does that mean for a character’s ability to lift something? How much can that character deadlift or lift overhead or bench press? How much can he lift and walk with (farmer’s carry)? If we’re saying that 11 is average, then let’s look at real world data concerning average ability in these categories, which are known and measurable, and let’s assign some numbers to an 11 Strength.
Since I use AD&D, this was done for me; I just tweaked the figures to make them a bit more logical and to get them in line with what I know about peak performance figures (from Olympic athletes and competitive lifters). Thus, I know that an 11 Strength character can deadlift about 220lbs while an 18 Strength character can deadlift about 1,200lbs. From this, I, as a real world person, could go to a gym and deadlift some weight, and use that to guess at my Strength score (between a 13 and 14). This isn’t the only measure, however, so to get a better understanding of my Strength, I would need to look at the other measures ~ bench press, overhead lift, standing high jump, etc. ~ to get an idea of my (real world) stats. (This probably brings my Strength down to an 11 because, frankly, I’m not that great with upper body work like bench press or pull-ups.)
But I don’t need to do that, do I? I don’t need to make comparisons to every day examples all the time; I just need to create something that my players can relate to. Something that they can use to gauge their actions in the game world, to predict how the world will react to their choices. Something that helps create an internal consistency to the game’s setting.
An interesting thing about this approach is that it doesn’t take into account levels. For the most part, attributes are static. They might improve with level, depending on your preferences, but generally they aren’t going up by a whole point every level. Hit points, on the other hand, are constantly improving. If we follow a similar methodology ~ define hit points first, then relate them to real world elements as a means of wrapping our heads around the concept ~ then we can adjust for the improvement aspect.
We know that hit points are a game convention. They exist as a means to describe the staying power of a character. They represent the total amount of damage a character can take before he goes down for the count. We also know that weapons have a damage value and that this represents the amount of hit points lost with a successful blow against a character.
These are the game elements. How, then, do they relate to the game? How can we describe them such that players benefit from that description, so that they can better judge the game world around them?
By first recognizing that hit points are relative. While an attribute like Strength is fairly static and measurable, hit points improve with every level; more importantly, damage does not. A sword deals the same base damage at 1st level that it does at 10th. Hit points, therefore, represent the relative skill of a character to absorb, ignore or push through damage. A 1st level fighter with 24hp is bothered by 4 damage from a dagger. A 10th level fighter with 90hp practically ignores the same blow.
Now this is the crucial part, and it’s a basic concept for the game that all DMs need to wrap their heads around: if I, as a player, can guess at a creature’s Strength by observing it performing certain tasks, then should I not be able to guess at a creature’s hit points by doing the same?
How can you have one but not the other?
I want to make sure I’m being absolutely clear about one thing in all of this: I’m not saying that players should know a creature’s stats simply by observing them. I’m saying that players should be able to guess a creature’s stats simply by observing them. I’m also saying that certain observations provide certain information – seeing a large creature uproot a small tree tells us something about its Strength while observing a six-legged creature run really fast tells us something about its Dexterity. Likewise, observing a creature fight another creature, or fighting the creature yourself and therefore having the chance to observe it firsthand, should tell the player something about that creature’s hit points. What, exactly, the player learns is debatable, but he can definitely learn something. What he learns tells him how the world works; it tells him how the rules interact with each other; it tells him how the DM interprets the rules; and all of this is useful information when the player is making his plans.
Because a DM who runs without care for logical, internal consistency is being a dick. He’s dangling a dollar bill in front of the players and saying, “just jump a little higher and you can have it,” but each time they try, he jerks it away from them.