Quests are a series of challenges that are accessible through the truck that you first see in each of the 4 main areas. Each quest offers prizes and items you can unlock.
There are 5 quests in each area. For the first 3 quests you complete in each area, you will unlock a picture that appears on the Quest Truck in place of the quest you just finished.
You get a ribbon sticker, free explore, and a special kart when you finish the quests in an area.