“Descriptive audio means that your video has additional audio content that describes aspects of the video that are purely visual and not accessible to blind or visually-impaired people. Usually, there’s a second audio track that contains the description. Viewers can listen to the second track along with the primary track.
Audio description or video description or descriptive video service are three ways to refer to the ability to insert information into a secondary audio track that will give blind and visually-impaired individuals access to visual content on the screen. When available, visual description should be included in any transcript of a video. Although the ability to add descriptive tracks to audio has been available for over 20 years, it is still relatively an unknown field.
Today the top online streaming content sites offer the ability for users to access an alternative audio stream at no additional cost to their users. For example, all of Netflix original content and more have audio description available, and iTunes currently has the largest library of titles available to download with audio description." -Web Access Team