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.
"Until they become conscious they will never rebel and until after they have rebelled they cannot become conscious." Winston speaks of the proles.
"Until they become conscious they will never rebel" meaning that the proles will never rebel until they realize (or become conscious) that they are being oppressed by the Party.
"..and after they have rebelled they cannot become conscious" meaning that once the Party is overthrown, it will still remain a mystery as to whether life was really better before the Revolution because all of the historical documents are false. What truly happened in the past is a complete mystery.
Hope this helps!