An appositive is a noun, a noun phrase, or a noun clause which sits next to another noun to rename it or to describe it in another way. (The word appositive comes from the Latin for to put near.)
Here are some examples of appositives:
Don't leave your shoes there, or my dog, Ollie, will munch them. (In this example, the appositive is Ollie. It is in apposition (as it's called) to my dog.)