What is a appositive describing?

An appositive is a noun or noun phrase that comes after another noun or noun phrase to rename or explain it. For example, in the sentence "The president, Joe Biden, spoke to the press today," the appositive "Joe Biden" renames the noun "president". In the sentence "The city of Chicago, located in Illinois, is a major metropolitan area," the appositive "located in Illinois" explains the noun "Chicago".