What does the following quote from 1984 mean?" Until they become conscious they will never rebel and until after they have rebelled they cannot...

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.