When you pressed the "Edit" button in the browser window, you asked to be taken to the source code for the object you were looking at. The web page knows the relative path to the source code for the object - relative to the root of the project tree. What you are being asked to supply in this dialog is the path to the root of the project tree.

After you supply this directory, it will be stored away so that the next time you press "Edit" in the documentation for this project, you will not be bothered with this dialog.