The Module Navigation panel allows you to see all the modules in your model in a single indented list, and navigate to them directly instead of having to go through each containing module. This is useful for large models with many different modules.
The module names are organized alphabetically within the containment hierarchy. If the module has a description, the description will appear within the list followed by the name in parentheses. Click on any of the module names to navigate into that module.
You can collapse lists of modules by clicking on the 
at the left and expand a collapsed list by clicking on the 
at the left (this will appear when a list is collapsed).