Death saves

At the start of your turn with 0 HP, roll a Death Saving Throw: a d20 with no ability modifier unless another rule adds something. A 10 or higher succeeds. Three successes stabilize you; three failures kill you. A natural 1 counts as two failures; a natural 20 restores 1 HP. Damage at 0 HP causes one failure, or two if it is a Critical Hit. Damage equal to your maximum HP kills you. Successes and failures reset when you regain HP or become Stable. A Stable creature at 0 HP stays Unconscious but stops making these saves until damaged.

Source: supplied Player’s Handbook (2024), chapter 1 and Rules Glossary. This is a paraphrased quick reference; specific features can override the general rules.