I just learned what it means. Basically, it means that God is independent, wholly whole without anything besides Himself. In other words, He doesn't need us.
I love that. It tickles me.
Because that means that something in God wanted to have us around. Something about His generosity, desire to share, to commune thought it would be a good idea. We're not logistically necessary or a utility. We humans are the result of a divine and desirous choice.