“Audio description (AD) is a narration of visual information in a video to provide context, clarify speakers, and articulate visual elements. 

Often, people can understand much of a video’s content merely by listening to its audio. However, if a video includes content that is only presented visually, such as on-screen text, graphics, or actions that are not obvious from the audio (like facial expressions or gestures), this visual information must be described in order to be accessible to people who are unable to see it. 

Audio description is essential for people who are blind or have low vision, but it also benefits those with certain cognitive disabilities who have difficulty understanding visually what is happening.” - Excerpt from Harvard University Information Technology. 

Read more about it here.