When you are reduced to 0 Hit Points, you can take a Reaction to delay the Unconscious condition. You can immediately move up to twice your Speed without provoking Opportunity Attacks and take an action and a Bonus Action. Taking damage causes Death Saving Throw failures as normal, and three Death Saving Throw failures can still kill you. Following this, you have the Unconscious condition if you still have 0 Hit Points.
Once you use this feature, you can't use it again until you finish a Long Rest.