Since Orwell puts a great deal of emphasis into the discussion of rats earlier on in the book, we can assume that they are going to play an important role later on in the novel. Nothing is wasted in Orwell's writing, and if he takes the time to explain something or include a particular element in the story, you know he has a legitimate reason for doing so. Pay particular attention to objects such as the rats, for they are symbolic of other things. The rats will later represent the power that fear has over Winston.
There's not a lot that we can assume, except for that for some reason, they are going to play a role in the story. It's one of those things that you take note of, and think, "Huh, that's kinda weird," but then move on, and don't really think about until it comes again later, and you realize, "THAT'S why he put it in." For some reason that isn't really explained too well, Winston is terrified of rats. He makes this quite evident when he is with Julia in their hideout, when rats surface. They do their best to block the holes and keep the rats out, but it is significant because we learn that Winston, a grown man, is mortally terrified of these scurrying rodents.
So, Orwell let us know that about Winston for some reason, which is not quite clear yet. We are let in on the fact that there is something out there in the world that he fears, greatly. We can assume that that fear will come into play somehow later on in the story, but we don't know how just yet. Orwell is very careful and meticulous in his plot though, and everything ties in somehow. So keep your eyes open for that later on: the rats become significant. I hope that those thoughts helped; good luck!