Web Design Glossary...
Audio Description
An audio description is narration, spoken out loud. It explains visual details. This allows visual content to be accessible to the blind or those with vision impairments. Audio descriptions of visual content is important if, for example, a video provides content that is relevant to the overall understanding of the video but is not available/ recognized through the default audio already present. For example an audio description can take a movie, and talk you through it. The narrator tells you what is happening on the screen what you can not figure out just from the soundtrack.