By setting this variable array to a different value, you can set
the speed at which GameMaker: Studio automatically scrolls a
background for you along the vertical plane. A positive value will
scroll from top to bottom, while a negative value will be bottom to
top and 0 stops all scrolling. if you have also enabled vertical
tiling, either through the room editor or by using the background_vtiled variable,
then scrolling the background will make it loop infinitely.
if background_vtiled = true
background_vspeed = 2;
The above code checks background0 to see if it is tiled vertically, and if it is it then sets the vertical scroll speed to 2.