What does the following quote from 1984 mean?" Until they become conscious they will never rebel and until after they have rebelled they cannot become conscious."
This is a much-quoted statement, viewed as a commentary upon the commitment of people to the status quo, even when that status quo is effectively abusive. Winston feels that the only solution to the repressive dictatorship of The Party is for the people to rise up against it, but he is driven to despair by the feeling that only through the act of rebelling will people become empowered enough to understand that they are repressed. The people are in an impossible situation: nothing but the act of rebellion will actually wake them up to the situation they are in, but they must be made aware of their situation in order to want to rebel . . . and so on, and so on.
Essentially, in the case of The Party, it is the element of mind control that prevents people from rebelling. Because The Party controls everything people think and feel, nobody is able to conceive of the idea of rebellion because it is not within the dictates of what The Party wants them to think.
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!