The location of your Godot Editor executable would be your installation folder, but it generally doesn't matter. Unless the installation folder is the same as your project root folder, in which case it will run the project instead of open the Editor.
Your project root folder is the one with a project.godot file, and arguably the most important. It is the same as res:// when running from the projects source, but res:// has the advantage of working even once exported in a .pck.
The root of the Godot Engine source would be the folder containing a README, etc, when you download the source code of Godot itself.
Finally, there is also the user:// folder. It's default location depends on the platform your are using (OS or web), and it can be used to store player saves.