An appositive is a NOUN PHRASES used to modify another noun phrase, for example:
"The opera Carmen".
"My wife Lucy".
"Our friend the mayor".
In those examples, "Carmen" modifies "the opera", "Lucy" modifies "my wife" and "the mayor" modifies "our friend".
Appositive modifiers are distinguished from the non-appositive ones by their ability to stand alone in place of the whole NP: instead of "They invited my wife Lucy", we could have simply "They invited Lucy".