Audio description (AD) is a form of narration which enhances access to a video, artwork, performance or live event for people who are blind or vision impaired.
Delivered during gaps in dialogue, it's what tells a section of the audience what is happening visually on screen that can’t be determined through dialogue or other sounds. For example, AD will tell the audience that the cop cut the wrong wire or that it was the good guy who was shot.
Without it, people are left to guess what is going on, or rely on family and friends to fill them in on the action.
Below is a clip from The Hunger Games with audio description.