I need to know why Russia and Ukraine are fighting. Tell as simply as possible?

So pretty much i just need to know why they're fighting. I really just don't know what's going on.

I heard somewhere that some Ukrainians are migrating to Russia? (if that's true can somebody explain why the Ukrainians are moving to Russia? what are the push and pull factors?)

Please keep the answers simple because i can hardly make sense of the many articles and videos I looked up.

Thanks :)