An appositive is a noun that it adds more information to the noun it is beside. For example, in these sentences the appositive is in bold.
The monkey, an orangutan, was fierce.
The orangutan monkey was fierce.
This is two nouns in a row. The word orangutan adds more information to the word “monkey” and further clarifies what the speaker is trying to say.
There are two types of appositives: words and phrases. The above is an example of an appositive that is one word. An appositive phrase looks like this (in bold).
The monkey, a bright orange orangutan, was fierce.
In this case, we have added information to make the monkey more interesting and to add detail.
Appositives are usually set off by commas, but not if the first word of the pair is not specific enough. Appositive phrases will have commas setting them off, as they do in the example, although sometimes appositives do not: CFO Aggie Smith spoke (CFO not specific; no commas); Aggie Smith, CFO, spoke (Aggie Smith, specific; commas). Appositive phrases have no verb and do not begin with a preposition; they have just a noun and words describing it.