Professionally
When you gain a Favor, each player, including you, puts 100 actual dollars on this. At the end of the game, the winner gains all of the money on this.
Digitally
If this is an in-person game, you may spend 5 Favors to move to the online version.
Physically
If this is an online game, you may spend 5 Favors to move to the in-person version.
Originally
When you play a card, you may first spend a Favor to instead play an earlier version of the card mentioned in the Secret Histories.