An appositive, also called an appositive phrase or appositive noun, is a noun phrase that comes after another noun phrase (called the antecedent) and provides additional information about it.
An appositive may provide essential identifying information about the antecedent, in which case it’s called a restrictive appositive and is not separated from the surrounding text by any punctuation (e.g., “the writer Djuna Barnes was …”).
The other kind of appositive is a nonrestrictive appositive, which provides extra information that is not needed to identify the antecedent and could be removed without changing the basic meaning of the sentence. It’s separated from the surrounding text with commas or other punctuation (e.g., “my hometown, Boston, is …”).