A modal dialog is a window which, once opened, will not allow the user any interaction with the window below until the dialog is closed.
In GNOME 3 the default modal dialog behavior is to attach the window so both the back and front window will be firmly attached together and dragging one of them will move even the other window, like if they became a single window.
When multiple windows are opened the desktop can soon become a mess, fulfilled with many windows covering the others and to find your application can be really frustrating. The GNOME Shell offers multiple possibilities to help the user to handle multiple windows.
The absence of a classic windows list like in others desktop environments could initially make you feel lost, it’s never easy to change your habits. After years of MS Windows, Apple OS X, GNOME and KDE the new GNOME Shell solution will surely appear unfriendly to adopt but after the initial approach it should you result really simple and natural.