So, how does the fediverse work?

I want to take a crash course in Fediverse. I was browsing stuff on Mastadon and voila! I found these communities on Mastadon but I wasn’t able to post anything, but I was however able to reply to stuff. I don’t know how this works. I won’t be reading it now, but I want to know more about Fediverse, so a book or...