Magazine = Subreddit. Exactly the same. Basically, a group/forum devoted to a topic.
Thread = Thread. Same as on Reddit.
Microblog = A feed of "non-thread" posts, usually stuff that comes from the rest of the Fediverse, but also from contributors here. This is what makes kbin special! Each magazine can define a list of hashtags that pulls content from elsewhere to complement what's going on on kbin.
Post = Something that will show up in the microblog. If you click "add post", your post will show up in the microblog, not in the Threads.