From the Super Mario Wiki
Flipside Tokens are special Rubee-like tokens that can be won by playing minigames in the Flipside Arcade in Super Paper Mario. Each Flipside Token is worth three Coins. By playing the minigames in the arcade, Flipside Tokens can be won and exchanged for rare prizes.
They are similar to Pianta Tokens from Paper Mario: The Thousand-Year Door. Once again, three coins can be traded for one Flipside Token, and three of the four minigames cost ten Flipside Tokens to play, the same amount as in Paper Mario: The Thousand-Year Door. The secret game, accessed with a Golden Card, costs twenty Flipside Tokens to play.
The player will earn one Flipside Token for every 10,000 points he or she scores in any minigame.