Welcome! Microblogs double as kbin's integration with mastodon. On kbin, it's a stronger upvote that boosts its visibility here, but it would come through to mastodon users as a retweet for anyone following you.
Only, kbin is an extremely young platform, so it's still a little awkward. You can follow users wherever but there is no dedicated feed to see what they post and reblog, for example, unless you go to that person's profile and look around under the post and boost headings. Which is a bit time-consuming. All that is still in the works.
That's also what "create new post" means, btw. Threads are kbin/lemmy posts. "Make post" is making a mastodon tweet. It's a thing to get used to.
Since "microblogs" is pulling content from mod-chosen mastodon tags, tweets will show up under the microblog section of any relevant community. But only if the creator thought to enter tags to look for, and only if the community was created on a kbin server. News@kbin.social has 2339 microblogs, but news@lemmy.world has zero because lemmy currently lacks this functionality.