expand_less Nothing is completely certain, luck can always turn the tide.
Whenever a player rolls a natural 20, they must roll again.  If the second roll is also a natural 20, then they roll again (critical confirmations count as the second roll).  If the third roll is a natural 20, then that player automatically wins the current encounter.  They gain narrative control of the story and may describe how their character managed to single-handed overcome the current challenge.  Players narrating should still be respectful to other players and the GM.
Similarly, if a player rolls a natural 1, they must roll again.  On another natural 1, they roll again.  In the event of three natural 1s in a row, that player automatically loses the current encounter.  Usually, this means that the party fails in its objective, not that the player's character dies or is otherwise removed from the action.
NPCs can roll Triple 20s too, but this only applies to named NPCs.  Any NPC can roll Triple 1s and lose the encounter.
In rare instances, a player or NPC can roll more than three natural 20s or natural 1s in a row.  These are handled on a case-by-case basis, but they could result in the character winning or losing the entire adventure / quest, gaining some lasting boon, or even dying terribly.