When you drop to 0 hit points, one of the following happens:

Instant Death

Massive damage can kill you instantly, if delivered in a single attack. If you take at least twice your maximum HP in damage, you die instantly. Otherwise, you fall unconscious.

For example, if you have 20 maximum HP, any attack that deals 40 HP or more in one hit will kill you outright.

Falling Unconscious

If damage reduces you to 0 hit points and fails to kill you, your character falls prone, incapacitated and unconscious. You can’t move, and can not take any action or reaction. You drop whatever you’re holding, automatically fail Strength and Dexterity saving throws, and all attack rolls against you have Advantage.

From the following turn onwards, you make Death Saving Throws until you are stabilised or until another character helps stabilise you.

Death Saving Throws

Whenever you start your turn with 0 hit points, you must make a special saving throw, called 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 any ability score. You are in the hands of fate now, aided only by certain spells and features that improve your chances of succeeding on a saving throw.

Roll a d20. 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 (see below). On your third failure, you die. The successes and failures don’t need to be consecutive; nothing happens until you accumulate three of one or the other. Keep track of both on your character sheet 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 1 or 20. When you make a death saving throw and roll a 1 on the d20, it counts as two failures. If you roll a 20 on the d20, you immediately regain 1 hit point and your consciousness.

Damage at 0 HP

If you take any damage while you have 0 hit points, you automatically suffer a critical hit. If this damage exceeds the rules for Instant Death as described above, you instantly die. It’s important that your companions try to prevent you from being attacked until you stabilise or you are healed up. If you suffer a critical hit but don’t instantly die, you remain at 0 HP, but you may be maimed, scarred, or otherwise permanently damaged from the blow, because of your vulnerability. If you later die after being maimed or amputated, these injuries may hinder your resurrection.

These attacks have no effect on the count of your Death Saving Throws.

Stabilising

The best way to save someone with 0 HP is to heal them with spells, potions or otherwise restore HP. This restores HP and the player regains consciousness regardless of any death saving throw successes or failures. If healing is unavailable, the player can at least be stabilised so that they aren’t killed by a failed death saving throw.

You can use your action to administer first aid to an unconscious person and attempt to stabilise them, which requires a successful DC 10 Wisdom (Medicine) check. This doesn’t restore any HP, and they remain unconscious, but no longer need to make death saving throws. If they remain stable, they regain 1 hit point and their consciousness after 1d4 hours.

However, once stabilised, if they take any damage, they will return to making death saving throws.

Resurrection

Perhaps you can find a powerful priest or wizard to resurrect your dead companions, provided you have recovered their body. The costs will surely be high. Rumour has it that magical items of great potency can also bring the dead to life. Finally, deities have been known to intervene in such matters… in return for a service.

Coming back from the dead is an ordeal. The target takes a -4 penalty to all Attack rolls, saving throws, and Ability Checks. Every time the target finishes a Long Rest, the penalty is reduced by 1 until it disappears. The target also loses 50% of their current XP to next level.