Can anyone explain to me what a Microblog is and how is it different from a thread? Are they similar to what a Toot is on Mastodon?

Microblogging is tweeting, or tooting, or similar. Very short messages, usually from one person and directed to whomever happens to notice them.

https://en.wikipedia.org/wiki/Microblogging

A thread is a collection of comments from people replying to one another, usually organized in a tree structure. It's more like a conversation.

https://en.wikipedia.org/wiki/Conversation_threading

That’s correct. Kbin offers both an ability to “toot” or an ability to create topical threads through its magazines

Microblogging is the name for things like Twitter. You "blog" only small texts like the famous 160 characters of Twitter. A tweet or toot would be called micropost or just "status update". On microblogs you usually don't have threads but replies to a post. These replies and the initial post effectively form a thread but all the replies are also separate posts by the user.