From the Super Mario Wiki, the Mario encyclopedia

First appearance

Dr. Mario World (2019)

Effect on player

Increases the score by 10%.

Score+ is a bonus booster in Dr. Mario World. When used prior to entering a stage, it increases the player's overall score by 10%, making it easier to obtain stars for a stage. Players can get a free first trial of the Score+, where subsequent uses of it require players to spend 10 diamonds or is earned when completing special events.