Typically, an "abstract" painting refers to subject matter that is real, but is pictured in a new/skewed/distorted way: aka - it is still a picture OF something. Non-objective abstract paintings, however, are not OF anything: they're just shapes, colors, lines, values, arranged in some sort of intentional way by the artist. It's not necessarily supposed to BE anything. Not anything realistic anyways. It could be an emotion, like love or anger, or something that has no visual representation, like music.