To provide more information or to identify a word, a noun or pronoun is placed with another noun or pronoun, referred to as an appositive phrase.
An appositive phrase may be nonessential or essential.
In essential appositive, the words are not set off with commas.
Whereas, nonessential appositive refers to the addition of meaning in a sentence which is already clear and is set off with commas.
Hence, appositive phrase is "a city on the Fox River" as it modifies the noun Aurora.