A background is a two-dimensional image resource. It is one of two visual resources offered in Game Maker, the other being sprites. The main differences are that it can't have animation, it can't be assigned to objects, but it can be used as a tileset.
ENIGMA stores backgrounds as RGBA bitmaps. They can be stored in the executable itself or be loaded externally on runtime. LateralGM can load any Java-recognized image format, such as bmp, jp[e]g, png, tif[f], and gif, as well as any additional SPI-registered format such as wbmp and ico. In the end, the format of the loaded file does not matter, as the storage mechanism is the same. This may change as the project shifts to allow external storage.
Backgrounds are usually used as scenery in the room. Room can be populated with backgrounds via the room editor. Background can also be split up into a tileset. They are small images that correspond to a part of the background. Then they can be placed individually in the room to form scenery. In a top down game these parts could form a road for example.
They can also be drawn from an object's draw event.