In that book, there are a series of worlds seemingly arranged in a sort of order of "perfection" or approximation of a central "world" (world in a loose sense). Using spaceships, it is possible to move between these worlds, "upstream", gradually approaching the perfect world through a series of less and less imperfect worlds (if you're curious and aren't planning on reading the books, more information is on these pages, although the jargon might be impenetrable:
[1] [2]).