From the Super Mario Wiki
Boosters are items in Mario Party: Island Tour. They appear only in Rocket Road and can multiply the Dice Block's value if used. If a player uses four Boosters and gets a 25, it results in an automatic win, due to the board's length.
Boosters are used to multiply the values on the Dice Block. Despite this, 0 always remains on the Dice Block. If the Dice Block lands on 0, the Boosters are used up, though they still receive a Booster if they are on a Booster Space or at the start of the board.