LibGDX is a game development framework that gives you a bunch of useful features (a 60 fps render loop, animation, input, physics, and lighting for example) and allows you to deploy as a packaged executable, an HTML/JavaScript page, or an Android or iOS app.
This multiple deployment feature is extremely useful. It allows you to write your code only once and have it work on pretty much any platform.
These tutorials assume you’re already familiar with the basics of programming and that you’ve already read the Processing tutorials and the Java tutorials. You should be familiar with object oriented programming. If you’re planning on deploying to Android, you should also probably read the Android tutorials as well.