Just as Closed Captioning helps the hearing impaired understand films, Audio Description (also known by the WGBH/ PBS-given name Descriptive Video Service) helps the visually impaired. This is aired on what is called a "secondary audio program" channel, or SAP.
What happens is there is a sound track that has a narrator describe the visual action of a video image, taking care to interfere with spoken dialogue and/or song lyrics as little as possible, which means a DVS narrator has just as big a job as a caption transcriber, if not more, to keep out of the action as a neutral observer. However, sometimes the narrator has to take an acting role such as reading out language subtitles of a character in character, ie in Star Wars: A New Hope, the narrator effectively acts out Greedo's dialogue in the Cantina scene.
Unfortunately, while some DVD publishers, like Disney, Paramount Pictures, and Dreamworks Animation, offer described soundtrack settings, the feature so far is nowhere near as common as subtitles or closed captioning, although organizations that assist the visually impaired are doing their best to offer videos with it. Also, the menus accessing Audio Description (and other audio tracks) are still entirely visual in presentation, so anyone who is blind or severely visually impaired often needs the help of a sighted person to access the feature. (However, some discs will now start the movie with Audio Description enabled if the user presses "5" on the remote control.)
One of the problems with DVS on television is that because only one additional audio track via SAP was usually offered in the analog age before television was switched to digital, DVS had to compete with foreign language translation tracks (mainly Spanish in the United States, French in Canada), and usually Networks want more viewers rather than provide a public service and preferred translation to description. Many stations didn't bother at all with SAP channels so they could avoid complaints from disgruntled viewers who changed to SAP accidentally about how some guy keeps butting in explaining the obvious, why Courteney Cox has a different accent and speaks Spanish, or why a mechanical voice is delivering forecasts to them (most stations with a SAP channel carry a local government weather radio service when DVS or translation isn't needed). This can be annoying when the feature is advertised by a network and isn't there for a viewer in their market.
An attempt by the FCC to mandate DVS in most primetime programming for the largest 25 United States television markets (in effect mandating it nationwide as said markets cover all time zones and networks wouldn't offer non-DVS feeds to other markets based on simple costs) was declared unconstitutional in the courts in 2002, and thus only in the digital age when multiple language tracks can be sent over a television broadcast or be placed on a Blu-ray has DVS been able to have a footing beyond PBS and Turner Classic Movies, the two biggest supporters of DVS. CBS's Police Procedurals also have had DVS tracks for many years.
However, Congress finally passed a little-known and publicized bill in 2010 to allow the FCC to do the above, and beginning July 1, 2012, a slow-phase in of required description began across the country until the top sixty markets have it in 2017, along with the requirements that the top five cable networks carry some series with description. The start of the 2012-13 season brought in a large surge in network-described programming, including most of Fox and ABC's sitcom lineups. Also helping was the happy side effect of all National Football League games getting Spanish SAP audio in 2012 as the league mandated it on all their broadcast partners, which pretty much gave a Trojan horse to stations to carry DVS tracks in order to keep the NFL's Spanish-speaking fanbase happy (DVS in most American "ball" sports would be redundant as the action is already described well enough in play-by-play; obvious number one cable network ESPN got an FCC waiver when the requirements started because of this, mainly to protect ESPN Deportes from any (even small) ratings losses from providing SAP audio in Spanish on the main channel.
In 2016, the FCC began to require that local stations have the DVS track automatically and audibly read out the text of any severe weather tickers that might appear on-screen to alert visually-impaired viewers of severe thunderstorms, tornadoes, and other weather incidents. The Weather Channel now also provides this same service. In the same year and 2018, NBC's primetime coverage of the Olympics was enhanced with live and acclaimed audio description. Procter & Gamble also has description on some of their ads, mainly on Charmin ads which seem to have a narrator who plays the straight woman to the antics of the toilet paper-loving bears she's describing.
Some networks, like Nickelodeon, maintain separate schedules listing audio described content on their respective websites.
Online, like closed captioning went through, has had several bumps in the road for described series. Netflix was the first to get into the act, starting logically enough with Daredevil (2015), which is about a blind superhero, with the rest of their original lineup getting description by the start of the summer of 2015, along with acquired series that already have the audio description bedded in. Currently, it and ABC's website and app are the only online television services providing description services (not counting cable provider 'watch live' portals). Hulu settled a lawsuit in the fall of 2018 involving their lack of audio description, and will introduce it, along with better navigation of their site for visually impaired viewers, throughout 2019. Many online film/TV stores also provide audio description tracks for their purchased titles.
In the U.K., many popular West End plays and musicals, as well as National Theatre productions, offer special performances where visually-impaired people are given headsets to wear that allow them to hear live audio description of the show in question, following on from the now-common practice of subtitled and signed performances for hearing-impaired audiences. This same system is used in most movie theaters which offer audio description of films in the United States and Europe, with the occasional 'open description' film showing where the auditorium sound features the description.