An item is an object a racer gets and can use when they drive through a rainbow box with a question mark in it, called an Item Box. These boxes can give many items ranging from defensive to offensive, some being very weak and some very strong. The strength of the item you gain is based upon what position in the race you are. For Mario Kart Wii, there is 12 spots available so a lot of spaces also allow for the same items just so it's not overly repetitive.
Fake Item Boxes should not be confused with Item boxes, they look the same but instead of giving you an item they will cause the driver to crash and be stunned briefly.