How do you use the gambling die in backgammon?

The die you are referring to is actually called a 'doubling cube'; it is one way in which the stakes can be raised in a backgammon match. The use of the doubling cube only makes sense when a backgammon match is played to more than one game.

The use of a doubling cube does not necessarily mean that a backgammon match is being played for money.