This is one of the ironies of the book since there is absolutely no chance that the Proles will ever be able to organize; they are so harmless that the Thought Police and other government agencies don't even both keeping an eye on them. Winston's statement that they are the only hope just shows how hopeless Winston's dreams are. He has the romantic illusion that the lower class has some kind of innocence, some essential goodness that will give them the will and power to revolt.
This is total nonsense, but Winston needs a dream. Keeping his journal and expecting not to get caught is another dream that he needs. It is only at the end of the novel that he learns that power exercised not for any ideal for for its own sake, isn't going to be beaten by any of his romantic dreams.
At this point early in the book, Winston realizes that no member of the Party can ever revolt. Between the heavy propaganda, the Thought Police, and the absolute willingness to spy on everyone's behalf, there is privacy, and no possible way to stage a revolt among Party members. That is why Winston says "Freedom is the freedom to say that two plus two make four. If that is granted, all else follows.” He knows that under this system, if the Party says 2+2=5, it will be accepted as truth.
In this society, the Proles (short for proletariats, the working class) are the only section of society with any form of freedom. They can say whatever they want, marry whomever they want, own whatever they want, etc. that is why Winston believes that they are the only hope of breaking the cycle of the Party's control. However, after visiting the proles' community, Winston realizes he is wrong. The world of Oceania is trapped in a paradox: the proles are the only people capable of revolt, yet they have no reason to do so. they are not bound by the rules of the Party, and they have desire to overthrow the ruling class. Thus the cycle continues.