D&D has struggled to find a good answer for "what happens when my character hits 0 HP?" The original answer "you die" leads to a lot of character deaths at 1st level, and eliminates a lot of potential drama with trying to recover/defend a downed and wounded ally, being wounded and captured, etc., so even groups who are comfortable with a high-lethality game have looked for alternatives. Conversely, many of the common alternatives, such as unconsciousness at 0 HP and death at -10 or 5th edition's death saves make the characters too resilient: character death is technically possible, but you need to work pretty hard to make it happen.
Various alternatives exist: a single saving throw at 0 HP is one perfectly good option. Two-tier HP systems, where characters have a lot of ablative HP (commonly called something like Grit, Sweat, etc.) and a small amount of "real" HP representing physical damage (Flesh, Blood, etc.), with death at 0 real HP, is another good one. Bucklers & Backswords rolls the chance of death into weapon type, which interacts nicely with Elizabethan surgery – in a rapier duel it's quite probably for the winner to end up bleeding out from a mortal wound like Inigo Montoya. Other games have adapted some technology from The One Ring and/or various PbtA-derived systems, where "Wounded" is a condition you can get and "Dying" occurs if you're Wounded twice. The most popular alternative in OSR games however is probably the Death & Dismemberment table.
