Federated Threads users still can't see posts from other fediverse apps yet. But you can see Mastodon user profiles if you get a like, follow, or reply and can choose to follow that person.
It's just the ok symbol. Tldr; it was what I would call a shitposting campaign on 4chan. You can read about it on Wikipedia under the "white power symbol" section: https://en.m.wikipedia.org/wiki/OK_gesture
Most of the time I would assume that people don't mean it as a white power symbol, but this asshole has been shitting up threads for a while now so I don't have much left for them in the way of charitable readings.