I'm not too sure but I will try. An abstract painting is a painting that isn't very clear to people or that doesn't depict a clear image (It doesn't make much sense to anyone but the person who drew it). When people remember stuff sometimes it isn't very clear, or if they try to explain it to someone else the other person doesn't understand it well, So all in all, I believe the author is trying to say that a memory is usually unclear or foreign to anyone but the person who had that memory.
