Demystify Java for me

I'll prerequisite the rest of this post by saying I know I'm asking the impossible here. :)

I've just finished the first 3 courses in the Java track. I'll be honest, I wasn't expecting to see how intense and involved they'd become. By the last video, I couldn't keep up with the code and I really can't say I've picked up all the skills that the course says I have.

That means diving back in again and learning the skills but there's a lot to catch up on.

I know the very basics. How to prompt for data and write data to the screen. How to import packages... how to display data through format specifiers. I know the basics of access level modifiers in classes, variables and methods. I can see that the code is class oriented and a lot of work goes in with "Interfaces". So we can use Maps, Lists and Sets to store and organise data. But that's as far as my understanding goes I'm afraid.

What do I need to learn to continue please on to the next course? (Local Development Environments... what should I spend more time looking at before I move on?

I want to say again that I'm not moaning about the content in these . I'm just slow to pick things up and things are not sinking in through no fault of my own. It's the curse of my brain I'm afraid.