An audio description is a narration of a video that describes the visual details that cannot be understood from the main soundtrack alone. Audio descriptions help people who are blind or who have low vision understand and enjoy the visual components of a video.
Actions, facial expressions, settings, changes in scene and environment, appearance of characters and any on-screen text are examples of content that is included in an audio description. These details are often announced when there are breaks in speech or soundtrack of the original video. The audio description works in tandem with the audio of the video, it does not replace the video's original narration & soundtrack.
Some video players have a toggle button that allows a user to turn the audio description on and off, similar to a caption on/off button. If that functionality does not exist in the video player used, then a separate version of the video with the audio description needs to be provided.
Here is an introduction to audio descriptions including a few examples of videos with audio descriptions.