You can obtain one or more items by breaking an Item Box. There are standard items that any driver can use, such as Bananas, Green Shells, and Mushrooms, as well as special items that only certain drivers can use, such as Mario's Fire Flower and Peach's Heart. You can find out more about items in the in-game Tips & Tricks in Help and Inquiries.
The chance of a particular item appearing depends on certain factors, including your position in the race when you break the Item Box, the amount of time that has elapsed since the race began, the lap number, and the items that you or other drivers are currently holding.
The special skills of gliders can increase your chances of getting a particular item. However, this increase differs depending on the situation. For example, the special skill that increases your chances of getting a Bullet Bill increases that chance when your position in the race is low.