What is "System Events"?
System Events is a faceless background app provided by Apple that gives AppleScript access to certain system functions and attributes. For example, we can use the app to change the desktop picture, control the user interface of other applications, change the screen saver, navigate the file system hierarchy, and more. Open Script Editor, choose File➔Open Dictionary..., and choose System Events from the list of apps. This will show you the AppleScript commands you can invoke.