OwnCast is a sort of Fediverse alternative to Twitch. It lets people host their own video livestream with a chat window at the side, and OwnCast accounts can be followed from Mastodon etc.
If you follow an OwnCast account from Mastodon, you will see a post from them in your timeline telling you when the account has gone live.
Also, if you follow an OwnCast account or boost its posts, this will appear in the livestream's chat window (see the attached picture).
Some OwnCast accounts you can try following right now include:
@live (soulful house music shows)
@hatnix (Linux gaming streams)
@Hamish - (retro gaming streams)
(Note that some OwnCast accounts do not have Fediverse switched on yet, so they won't be visible on Mastodon etc)
the personal instance of Liaizon Wakest