Quite frankly - it’s everything and nothing in particular.
Bubble is so wide that every little bit (databases concept, design basics, concept of programming functions methods, etc.) helps!
But the beauty is that you don’t really need to KNOW any of that to get started.
Bubble is a wonderful way to help you to help to wrap your head around a lot of these concepts and much more.
So the best suggestion is “just do it”.
You will understand on the way of what exactly you’re missing in terms of specific framework or module.
And as many pointed out - forum is THE place for finding the answers.
Don’t get stuck in analysis paralysis 
just do it!
P.S. And of course, there is a ton of resources to help you with it. Both paid and free.
Here’s a great reference topic.
http://forum.bubble.io/t/list-of-bubble-tutorials/50291/2?u=vladlarin
Vlad Larin
