Cscope will help you understand the program by assisting you in finding the usage and definitions of variables and procedures. You supply it with the list of files it needs to search. It then provides search capabilities with a variety of simple methods.
Cscope is curses based (text screen). An information database is generated for faster searches. The fuzzy parser supports C, but is flexible enough to be useful for C++ and Java. It supports a command line mode for inclusion in scripts or as a backend to a GUI or other frontend.
Cscope provides methods of searching for:
Once found, you are shown a scrollable menu of places where the item in question was found. You select the the item and your selected editor is started on the line where the selected text was found. When configured to open editor sessions in independent windows, looking through all the places where a variable gets used becomes a humanly manageable task.