ELI5? McDonald's and Burger King. One has a Big Mac, the other a Whopper. One has red and yellow, the other red and blue. Either way, you're getting a burger.

Oh, they're also right next to each other so you can wave to the people in Burger King from McDonald's and vice versa. Now everyone is enjoying burgers together.

lemmy is the more mature platform, whilst kbin is newer and more feature rich.

What it amounts to is that kbin can do things that lemmy can't do, but the things that they both do, lemmy tends to do better. And as kbin is effectively in alpha at the moment, it doesn't have much documentation, making installation and configuration a challenge.

The biggest point of difference in features is that kbin is aware of other fediverse content in a way lemmy isn't. kbin and lemmy both talk to each other really well, but kbin also natively supports other types of fediverse groups (gup.pe, friendica and chirp). kbin also lets you see non threadiverse content, by attaching hashtags to groups. So if you set up say a cycling group on kbin, you can also make the group watch the #cycling tag, any any mastodon or other micoblogging content will appear on a special tab in your cycling group.

Kbin is also written in PHP where as Lemmy in Rust, nuff said

Lemmy and kbin are two different forum software that can be installed and run on servers. Because both use the ActivityPub protocol, the content between them can be shared. So, a Lemmy user will be able to see content from a server running kbin, using Lemmy.

Okay, so think of every website that is part of the "Fediverse" (aka uses "ActivityPub") as just being different ways to display the exact same data. Sometimes their data works really well between two of them and sometimes it's a bit more awkward. Lemmy and Kbin are both trying to imitate the "forum-style" UI that Reddit uses, so they integrate really well with each other. Same data, slightly different UI. Mastodon, on the other hand is imitating Twitter. So trying to read Mastodon in Lemmy is like trying to read a Twitter feed as Reddit threads. It's messier. Kbin seems to be trying to find a way to better display Mastodon-style threads within their UI. Otherwise, I think the big picture way to understand the difference is just that it's a matter of UI and which one you prefer more.

Kbin’s built in php which makes it less attractive to contribute to imo

It is the difference between nginx and apache: two pieces of software that do basically the same thing. With the exception of some naming conventions and UI differences, they are the same and both participate in the community in the same way.