Appositives are also known as parenthetical statements, and they are usually denoted by commas in the middle of a sentence. They usually contain information that, while it is relevant, isn't 100% necessary for the sentence to have. In this situation, the appositive is "either Juan or Linda," because it is separated by commas, and again, it is relevant, but the sentence would be just fine if that phrase was omitted; it would just be a bit more vague as to whom the sentence is referring.
