This quote shows how bad Winston feels about the chances for change in his society. In this passage, he is talking about the likelihood that the Party will ever be overthrown. He has decided that the Proles are the only group with enough strength to overthrow the Party, but (in the quote you cite) he is saying that they cannot really do this.
The reason for this is that the proles are stuck in a paradoxical situation -- a "Catch-22." They cannot rebel until they are conscious (until they realize that they are being abused by the Party). But, at the same time, they cannot understand they're being abused (in Winston's opinion) until they have rebelled.
This means that things are hopeless -- the proles can't rebel until they've already rebelled. This is obviously impossible. This thinking shows how deep Winston's despair is.