kbin and lemmy are different softwares, but they are both used for link aggregation and the 2 softwares use a common protocol, so they can talk to each other. So there are kbin servers and lemmy servers, and they are all interconnected.
So now we can take this post as an example:
You are a user on kbin.social
You posted this question on /m/[email protected] - this means the community you posted on is actually hosted on lemmy.world
lemmy.world then tells other instances that its federated with that someone just made a post on /c/nostupidquestions on its instance. what kbin calls magazines are called communities on lemmy, hence the /c/ instead of the /m/.
kbin.social and all the other instances will then also show this post, even though it originally was created on a different instance