The File > Open command activates the Project Window and sets the focus on the text box of the Project Window. If the Project Window was not visible, it is made visible, and then hidden after you select a file to open.
You can customize what the Open command does in the Options > Preferences: Files dialog box. Instead of activating the Project Window, you could have it bring up other dialog boxes or windows instead.
If you do not have a project open, then this command will bring up the system Open dialog box, which is a standard system dialog box.
You can also open a file by dragging a file and dropping it onto the Source Insight application window.