Audio description is used to describe things that are happening on the screen. In this way the blind and poor sighted are able to enjoy the program (or event). A narrator provides extra information between dialogs and background noises about the video you are watching.
There are prepared audio descriptions and live audio descriptions. Prepared audio descriptions are recorded in a sound studio and added to the video. In the case of live descriptions, the audio describer is present during the event, giving live commentary.