Guidelines for time-based media, in other words audio and video content, are the only new bunch of guidelines in WCAG 2.0. Others existed before in a different form.
Other content doesn't change or move as you watch or hear it. Instead, your eyes move across print and images as you absorb information.
But audio and video travels past your ears and eyes, and you have to catch it on the hop.
This produces a different accessibility challenge. For various reasons, some people can't perceive your audio or video.
You need to think first:
At the most basic level, every audio or video file needs a title that describes and identifies the content. At least then people know what they're missing.
Giving people a valid alternative experience can be a bit more complicated. For example:
When you want to include audio and video in the content you provide, you need to be aware of these guidelines. Get help from your web or video team.