Altdrag changes the way you drag windows over the screen. When it is running you only have to keep Alt pressed and click on any part of the window you want to move and, keeping both Alt and left click pressed, drag the window to the place you want.
This function is specially useful in computers with a small screen, because sometimes it's hard to click on the little title bar.
If you press the right click, you'll change the size of the window. In short, Altdrag is an application that at first can seem to be useless, but those users who own a netbook will find it really helpful.