A driver's special skill allows them to use a special item, such as Mario's Fire Flower and Peach's Heart.
A kart's special skill increases the effect of actions like Rocket Starts or Mini-Turbos and the points gained from performing them.
A glider's special skill increases the chances of getting a specific item and the points gained from using it. This point increase only applies if the specific item appears in one of your item slots. The increase does not apply if the specific item appears as part of another item (such as Lucky Seven or Triple Mushrooms) or if it is dropped and then picked up on the course.