Simple. They are blood oxygen.I think "HP are not meat" is the biggest sticking point for a lot of people. Reason being: if HP are not meat, then why do you die when they run out? Since you die when you run out of HP, clearly they're meat...or at least meat adjacent.
Or blood glucose.
Or maybe just blood?
Oh wait, what about things like poison, disease, or radiation damage? HP must be a measure of the integrity of the DNA and cytoskeletons of all your cells. Yeah, that's it.
Or...maybe it's an abstract meter determining how long you're "allowed" to stay in the fight, because anything more involved gets complicated fast.
ObPython:
"'Tis but a scratch!"
"But we didn't eat the salmon mousse!"
"Can we have your liver?"