A death saving throw is a special kind of roll to see if your character dies. After your character reaches 0 hit points, they fall unconscious becoming Incapacitated. At this point, they start making death saves.

How is death saving throw calculated?

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 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.

How many death saving throws do you get?

Also note that you only make 1 death save per turn, so it takes a while before you can even become stable (usually 4-5 turns).