A Single Elimination bracket is one where once you lose your match you are out of the tournament.
A Double Elimination bracket is one where when you lose once you are sent to the lower bracket. In the lower bracket if you lose you are then out of the tournament.
In a double elimination bracket’s Grand Finals there will be one person from the Upper Bracket (with no losses) and one person from the Lower Bracket (with one loss). The winner of the tournament is the last one that has not had two losses in their record.