Complex sentence: In user interface design, a modal window (often referred to as an "extended drop down menu") is a graphical control element subordinate to an application's main window which creates a mode where the main window can't be used.
Split into simpler sentences: In user interface design, a modal window (often called an "extended drop down menu") is a graphical control element subordinate to an application's main window. It creates a mode where the main window can't be used.