Threadcasts are really just a live chat environment which initially were used to accompany broadcasts like sports matches, events or X Spaces. They've since become used for general chat around a particular topic (without an attached event).

It's really just a regular thread that someone has tagged # threadcast which means the chat updates every few seconds and by default is most recent first so has a slightly different feel. Ideal to accompany events but doesn't have to, as you can just use it to post loads of info on a given subject and hopefully have people join in...