This tutorial assumes you're comfortable with basic programming in Processing. You should understand variables, methods, for loops, if statements, and the basics of OOP.
Processing is great for making small games and visualizations, but for applications and more complicated things like libGDX, we need to use Java. Processing is a perfect first stepping stone, but "real" programs require Java.
This tutorial will go over the basics of Java. By the end of it, you'll be able to create a simple text-based game!
Next: From Processing to Java