A slightly more detailed explanation (from CodeLaunch) with some additional info at the end.
Some useful navigation shortcuts:
IntelliJ IDEA Code Navigation
This video (from LaunchCode) gives a pretty good explanation of how to use the IntelliJ IDEA debugger.
This video explains how to automate the 'Extract variable' refactoring using IntelliJ IDEA. Most other refactorings available work similarly. i.e.
select the code to refactor →
find the refactoring in the context menu or
use the keyboard shortcut.
Here's another video explaining how to do some more useful refactorings in Intellij IDEA.