When you play a game with real money, you play with the money you have deposited on your account. It is possible to withdraw your winnings in this game mode.
When you play a game in demo mode, you play with fictitious money. This can be useful if you want to try a new game before staking with real money or if you simply want to practice. It is not possible to withdraw your winnings in this game mode.
Note that you can switch from Real money to Demo mode at any time by using the "Play demo" switch :