It's roughly equivalent to using Outlook versus Thunderbird for your email. Same protocol, same ability to interact, but different codebase, slightly different interface, and possibly a few tweaks around the edges where the protocol itself doesn't demand a certain way of doing things.

So, for instance, a "!" link in Lemmy doesn't work in kbin, but remove the exclamation point and it will be fine. A Lemmy community is identical to a Kbin magazine. Properly configured and federated, a Lemmy and a Kbin instance are completely interoperable with each other. Kbin has the "microblog" tab that integrates it better with Mastodon, but I haven't seen a lot of discussion around that part of things, since link aggregation is driving the current increase in users.