When using multimedia, we must provide means for everyone to consume the media. Multimedia is anything that uses audio and video.
Videos with audio require synchronized captioning. Captioning can be either closed, meaning they can be turned off, or open captioning, meaning it is baked into the video. Anything said in the video must be included in the captioning, and anything written in the captioning must be said in the video. This includes names, sounds, and descriptions. In other words, the captions and audio should be equivalent. WCAG requires pre recorded video to be captioned at A, while live video requires captioning at AA.
Audio description ensures any information displayed visually is conveyed via audio. This can be done a couple of ways. The script for the video can be written in a way that all visual information is described by the narrator. The other way is to add a separate audio track that describes the visual content. This can be done with a special player or a separate version of the video with the audio baked in.
Some examples of when audio descriptions should be used are scene transitions, characters, actions and text on screen.
Just a note about keyboard access. All video controls should be accessible via the keyboard, but it's also worth noting that time stamp information should be available to screen readers as well.
Great page for videos and 508 Creating Accessible Videos created by the University of Washington.