Backgrounds

A background image is an image drawn or loaded into the game to be it's backdrop. Background images are particularly important in platform games. Backgrounds are generally used as images to better the look of a game quickly, by providing the player with something visually pleasing, but which they will never actually access.

For the full article, read Tilesets

Another thing which belongs to the background resources are Tile sets. With tile sets you can make Game maker show objects on the background, so you don't have to create a complete new background for each room, and you don't have to create different objects from sprites.