An apposition is a grammatical construction in which two elements, normally noun phrases, are placed side by side, with one element serving to identify the other in a different way.

My friend, a talented musician, played the piano at the concert.

The two appositions in the sentence provide additional information about the subject.

She explained the concept appositionally.

The appositional phrase, 'a famous author', provides additional information about the noun 'John'.

My sister, a doctor, is very busy.

The appositionals in this sentence provide additional information about the subject.

The appositioned words provide additional information about the noun.