When a creature drops to 0 Hit Points, it either dies outright or falls unconscious, as explained below.
Instant Death. A monster dies the instant it drops to 0 Hit Points, although a Game Master can ignore this rule for an individual monster and treat it like a character. A creature dies if its Hit Point maximum reaches 0. When damage reduces a character to 0 Hit Points and damage remains, the character dies if the remainder equals or exceeds their Hit Point maximum. For example, if your character has a Hit Point maximum of 12, currently has 6 Hit Points, and takes 18 damage, the character drops to 0 Hit Points, but 12 damage remains. The character then dies, since 12 equals their Hit Point maximum.
Falling Unconscious. If you reach 0 Hit Points and don’t die instantly, you have the Unconscious condition until you regain any Hit Points, and you now face making Death Saving Throws.
Death Saving Throws. Whenever you start your turn with 0 Hit Points, you must make a Death Saving Throw to determine whether you creep closer to death or hang on to life. Unlike other saving throws, this one isn’t tied to an ability score. You’re in the hands of fate now.
Three Successes/Failures. Roll 1d20. If the roll is 10 or higher, you succeed. Otherwise, you fail. A success or failure has no effect by itself. On your third success, you become Stable. On your third failure, you die. The successes and failures don’t need to be consecutive; keep track of both until you collect three of a kind. The number of both is reset to zero when you regain any Hit Points or become Stable.
Rolling a 1 or 20. When you roll a 1 on the d20 for a Death Saving Throw, you suffer two failures. If you roll a 20 on the d20, you regain 1 Hit Point.
Damage at 0 Hit Points. If you take any damage while you have 0 Hit Points, you suffer a Death Saving Throw failure. If the damage is from a Critical Hit, you suffer two failures instead. If the damage equals or exceeds your Hit Point maximum, you die.
Stabilizing a Character. You can take the Help action to try to stabilize a creature with 0 Hit Points, which requires a successful DC 10 Wisdom (Medicine) check. A Stable creature doesn’t make Death Saving Throws even though it has 0 Hit Points, but it still has the Unconscious condition. If the creature takes damage, it stops being Stable and starts making Death Saving Throws again. A Stable creature that isn’t healed regains 1 Hit Point after 1d4 hours.
Knocking Out a Creature. When you would reduce a creature to 0 Hit Points with a melee attack, you can instead reduce the creature to 1 Hit Point. The creature then has the Unconscious condition and starts a Short Rest. The creature remains Unconscious until it regains any Hit Points or until someone uses an action to administer first aid to it, which requires a successful DC 10 Wisdom (Medicine) check.
SRD 5.2.1 · CC BY 4.0