Audio Description is when visual information is described by someone speaking. Sometimes this is provided by a specialist audio describer and sometimes speakers will self-describe for the benefit of anyone unable to see them.
Each event may have a slightly different version of Audio Description:
For some events, an audio describer will be assigned to a person who cannot see the event, and will provide 1-1 audio description for them. Sometimes this might be done via a head-set during in person events, or via a separate audio channel during online events.
Other times, audio describers will verbally communicate to everyone attending.