Abstract paintings are in progress; they basically have no boundaries in the real world so anything goes. One way to begin to understand an abstract is by seeing it as the basic "skeletal" or "underlying structure" of what would become a finished painting of say, a landscape. It can be seen as an unfinished painting that is in its planning stage; where, hopefully, a strong composition has been developed and colour principles are followed. Good use of colour and design or composition is what makes a good painting, so once that has been established the painting can be said to be finished without it being representative of anything. Does that make sense?