The module browser shows you the structure of all of the files in your program. It can be opened via the Show menu, or via the Module Browser ... menu item in the Scheme menu.
A module browser window contains a square for each module. The squares are colored based on the number of lines of code in the module. If a module has more lines of code, it gets a darker color.
In addition, for each normal import, a blue line drawn is from the module to the importing module. Similarly, purple lines are drawn for each for-syntax, for-template or for-meta import. In the initial module layout, modules to the left import modules to the right, but since modules can be moved around interactively, that property might not be preserved.
To open the file corresponding to the module, right-click or control-click (Mac OS X) on the box for that module.
The module browser will also show you the phases that each module is loaded in; choose the “Long, with phases” menu item in the “Names” popup menu. The integers indicate the phases and if #f is present, it means the module is loaded for-label.