tl;dr: It's Mastodon. You can use Mastodon from Kbin.
The Microblog tab shows posts from Kbin + Mastodon, just like how the Threads tab shows posts from Kbin + Lemmy. So if you have people you like on Mastodon, search for them using the little magnifying glass and then follow them from Kbin. Their posts will appear in the Microblog tab.
Additionally, if people on Mastodon use a #hashtag for something, it'll automatically be sorted in magazines that care about that hashtag. This means if you follow someone and that person uses a hashtag, their post will be placed in the "Microblog" tab of whatever magazine relates to that hashtag. This allows other people to discover the person you're following through shared interests.
For example, the Internet Archive has a Mastodon account - @internetarchive. The Internet Archive used their Mastodon account to make a toot on Mastodon: https://mastodon.archive.org/@internetarchive/110611348840969515
Because people are following the @internetarchive account from Kbin (which you can do by going here or by searching Kbin for their username, @[email protected]), that post was federated here to Kbin. Once it showed up, it appeared in the "Microblog" tab and got automatically organized into Kbin's @Futurology magazine because it used the hashtag #research: https://kbin.social/m/Futurology/p/565801/Exciting-news-Introducing-ARCH-Archives-Research-Compute-Hub-a
Magazine moderators determine what hashtags they want included in their magazine - so @Futurology has said "We would like all posts with #research to show up in our microblog section". (You can go to @Futurology directly to see what hashtags the mod team thinks are relevant.)
If no hashtags are used on a post (or none of them match any magazines), then it goes to the magazine @random.
Wanna write a tweet/toot from right here on Kbin? Put it in a microblog. Use #hashtags to organize it into a magazine, or use the dropdown on Kbin to pick a magazine manually.
People can follow your Kbin profile from Mastodon. They'll see microblogs as a Mastodon toot, and "boosts" as basically Mastodon's version of retweets. People on Lemmy don't see boosts, but will see microblogs as a "normal" Lemmy post (since Lemmy doesn't have a "microblog" tab).
For example. I made this post to the Microblog here on Kbin: https://kbin.social/m/Disneyland/p/510705
I made it from the main "microblogs" tab, but it used the hashtag #Disneyland. @Disneyland listens to that hashtag, so Kbin automatically put it there: https://kbin.social/m/Disneyland/p/510705/Went-to-Disneyland-over-the-weekend-and-went-on-RunawayRailway
If you go to Mastodon, you'll see it as a normal toot: https://sunny.garden/@[email protected]/110587107756032019
If you go to Lemmy, you'll see that microblog as a Reddit-like post: https://lemmy.world/post/418494?scrollToComments=true
(Note that it seems things which come from Mastodon don't get automatically sent to Lemmy - just microblogs from Kbin itself. That Internet Archive post I mentioned above doesn't seem to exist on Lemmy.world.)
This behavior is one of the main reasons why I chose Kbin over Lemmy; I love that I can post once and have my stuff federated everywhere else super cleanly and easily. Lemmy is a bit more messy when it comes to Lemmy -> Mastodon and the devs aren't interested in changing how it works (I asked before I came over here).
Ernest seems really invested in playing to the strengths of the fediverse, and the Kbin roadmap has him planning to integrate more fediverse services in the future. For example, Mobilizon support is planned, which is like a group calendar on the fediverse.
If @Starwars wanted to have a watch party for a new episode of The Mandalorian, they could (theoretically) schedule an event on Mobilizon and have it federate to their magazine as a normal thread. Then they could (theoretically) pin the Mobilizon thread and use the comment section of the event as a Kbin megathread when the episode airs. See https://demo.mobilizon.org/ and imagine it being part of Kbin, just as Lemmy and Mastodon are "part of Kbin."