Activitypub is the standard all the fediverse application use, and that's what makes the various apps cross compatible (you can reply to a Lemmy thread from another Lemmy instance, or from a kbin one or even from Mastodon).
This standard is used by various WebApps to provide social networks experiences akin to the big centralized networks.
Lemmy and kbin work like reddit.
Mastodon and pleroma do microblogging (default message length is 500 char, can be modified on each server though).
PeerTube is a distributed YouTube alternative, again using activitypub to federate.
Now thread is a new microblogging app by Instagram and it happens to use activitypub, which means it could be interconnected with fediverse apps. You could in theory follow and interact with its users from any of the fediverse apps. But this feature is not on from their side at the moment. And many fediverse server admins plan to block federation with threads.net because they don't trust Instagram / meta.
What could happen is what did happen to XMPP / jabber.
Google embraced it for its instant messaging app, then after it had become the provider with the largest user base of said standard, it simply changed the underlying tech it used and stopped using the standard. To google users, their weird contacts that were not on the google ecosystem simply vanished, but they were a small number of people. To the users of xmpp servers, many of their contacts on the google side of things became unreachable again.
Now that's a thing that can happen if threads.net becomes the largest fediverse instance over time and loosing access to these users is a big detriment to users of other instances.
However, I couldn't care less if I lose access to the posts of threads.net users. I don't intend to interact with them at all. I don't use any mainstream social media at the moment, and I don't miss the ads, the drama, and the "normie" users, for lack of a better term.