This effect moves an element. Effect.MoveBy is older name.
For more informations about options see Core Effects (part about Common parameters)
This will move object to corner of the window (x=0; y=0):
new Effect.Move (obj,{ x: 0, y: 0, mode: 'absolute'});
This will move object 30px up and 20px to the right (the default mode is ‘relative’):
new Effect.Move (obj,{ x: 20, y: -30, mode: 'relative'});