The placing in immediately succeeding order of two or more coordinate elements, the latter of which is an explanation, qualification, or modification of the first. This syntactical unit is often set off by a colon or commas.
Inserting more info about a topic... Rudolph had a very shiny nose. Sentence right? Yes, but more info is needed so add an appositive phrase!
Rudolph, the red nosed reindeer, had a very shiny nose.