By setting this variable array to true you can tell
GameMaker: Studio to tile the background image for the
selected background along the vertical plane. This will make any
moving backgrounds appear infinite, or simply repeat the same image
over the height of the room if it is a static background. Setting
this to false will disable tiling.
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.