Every entry with [], like [DEFAULT] or [KEY_X] represents one payload you want to load, DEFAULT is the default payload loaded without pressing a key or an undefined key, and KEY_X for defining a payload loaded when key X is pressed.
For every payload you have multiple options you are able to set.
"path" in excample defines the location of your payload, based on the root of you SD-card.
"payload" defines the payload type, while -1 means that it should be automaticly detected.
"splash" defines the type of the play, based on the values defined
here.
"splash_image" defines the path of the image or animation file you whant to use before a payload is loaded.
For more informations check out the
wiki on github, espacialy
this page