What is the purpose of the Call Stack window in Visual Studio?
Each time you call a method, an entry is place on the "stack" for that thread describing the method and the parameters used to call the method. When the method returns, the method and it's parameters are removed from the stack. That's how the operating environment knows where to return when a method finishes. It just removes the top entry from the stack, cleans up any local variables that were created during that stack frame, and returns to the previous method. (That's over simplified, but generally the idea.)
You can think of it literally as a "stack" of the instructions that got you here.
That's what it means to the operating environment.
To the developer, the practical purpose is to help you understand why your program is in the state it is in. Whenever execution of the program stops in the debugger, either by breakpoint or by an exception being thrown (depending on your Visual Studio settings), you will have access to the current stack. Remember that this stack doesn't show ALL methods that have been called up to this point. Any method that completed was removed from the stack. It's not a log.
You can double click on any of the entries in the stack to go to that source code (if it's available on your machine). While you're there, you can inspect local variables, etc. It's a kind of detective tool to help you figure out what has happened in your program up to this point.