From the Super Mario Wiki
Jackpot+ Spaces are a rare space appearing in Mario Party 9. They only appear on the board Bowser Station, and they add the number of Mini Stars printed on the space to the Jackpot Machine in the center of the board. There are two variants of this space: a 5 Jackpot+ Space and an 8 Jackpot+ Space. When the machine reaches twenty Mini Stars, a Free-for-All minigame will be initiated to win the Mini stars.