If you've attached a video file to your post, it's a good idea to add a text description of what's going on just like you would for an image, so that blind people know what's in it through screen readers.
However, if your video has sound, remember to also describe the audio as well.
Describing sound in captions can be useful to people who are deaf, or those who are in a situation where they can't listen to the sound (on the bus, in a meeting etc).
On Mastodon you can add descriptions to files by attaching them and then clicking "edit" on the web interface, or simply writing on top of them in the official app.
Other Fediverse platforms and apps also support adding text descriptions.