Audio Description involves the accessibility of the visual images of theater, television, movies, museums and other art forms for people who are blind, have low vision, or who are otherwise visually impaired.  It is a narration service (provided at no additional charge to the patron) that attempts to describe what the sighted person takes for granted -- those images that a person who is blind or visually impaired formerly could only experience through the whispered asides from a sighted companion.

Audio Description is commentary and narration which guides the listener through the presentation with concise, objective descriptions of new scenes, settings, costumes, body language, and "sight gags," all slipped in between portions of dialogue or songs. 

(definition provided by the AudioDescription Project.)