Death Saving Throw
A Death Saving Throw is a special type of Saving Throw that a Downed creature must make to avoid dying. Typically, Death Saving Throws are only performed by player characters, Companions, and certain notable NPCs. Most enemy creatures simply die when reduced to 0 Hit Points.
Rolling Death Saving Throws
When an eligible creature is reduced to 0 Hit Points, they fall unconscious and become downed. While downed, the creature must use their full turn to roll one Death Saving Throw per round.
The die rolled is a 
d20 and the Difficulty Class of the roll is 10. Any bonuses to Saving Throws from effects (e.g. Bless or Aura of Protection), or bonuses from items that add to Death Saving Throws (e.g. Family Ring), but not bonuses from items that add bonuses to Saving Throws (e.g. Safeguard Shield) are added to the roll. A roll of 9 or lower results in one failed save, and 10 or higher results in one successful save. A roll of 1 results in failure, regardless of the modifiers to the roll.
If the downed creature rolls a total of three failures, they die. If the creature rolls three successes before rolling three failures, they become Stable - though still unconscious - and no longer need to make Death Saving Throws.
Healing a Downed Creature
At any time while downed, if a creature is Helped or healed by another creature, they immediately lose the downed condition, return to consciousness, and no longer need to make Death Saving Throws.
Note that granting a creature Temporary Hit Points is not the same as healing them, and does not remove the downed condition.
Receiving Damage when Downed
Whilst downed, if a creature receives ranged damage (either magical or martial) they automatically fail one death saving throw per damage application (i.e.3 separate Magic Missile stacks would automatically fail 3 saves and be considered dead.
Reviving the Dead
A dead creature cannot be healed by normal means and can only be resurrected by powerful magic, such as Revivify.
Gallery
An example of something impossible
