If you're posting a video on PeerTube, it's a good idea to use single word or CamelCase tags.
These kinds of tags will be visible not just to PeerTube servers, but Mastodon and other Fediverse servers too. This will make it much easier for people elsewhere on the Fediverse to find your video.
If you use tags that have spaces in them, these tags will not be noticed by Mastodon, so the video will be harder to find.