An appositive is a noun or pronoun that explains the noun or pronoun it follows.
An appositive phrase is made up of an appositive and its modifiers.
Most of the time set apart from the rest of the sentence with commas, but if the appositive is necessary to the meaning of the sentence or is closely related to the word it follows, no commas are necessary
1. Susan, the girl in the front row , is reading a book.