Jackpot Dollars (or GBP) are for use in the "Jackpot Battle". When you sign in, a message will appear and tell you what date and time the "Jackpot Battle" starts. Once you click on the message and enter your bank details, you will have a chance at winning your number of jackpot (GBP, USD, etc.), in real cash. These battles are not to be taken lightly and are very difficult to win.
You can collect Jackpot Dollars in bonus boxes, and also by completing certain quest strings (which also includes acivities such as "Spaceball").