Generally, the visualizer is a good tool to use if you have simpler scripts that you'd like to understand. Some of the questions it will help you with are: "How does recursion work?", "How does a function call work?", "How do various methods, such as append, work when working with lists?".
In order to start the visualizer, open a python script that you're able to run. In the menu, there will be a link with the title "Visualize". Click the "Visualize" link.
The next screen you'll see is the following:
The code (#2) is listed to the left. As you step through your program, you'll see two arrows in the left gutter that will show you what line has just executed (green), and what will be executing once you click "Forward" (red).
The right column displays the stack (#3) as your program executes. You'll see your variables getting assigned, objects being created, etc.
The bottom pane is your program output (#4). Anytime you have a print statement, or any other type of output, you'll see it displayed in this section.
Here is a screen shot of how it looks while stepping through a program:
Credit to the visualizer is given to Philip Guo at pythontutor.com. We forked, tweaked, converted, and modified his open source creation to work as a streamlined feature within the WebIDE.