What does Winston believe that proles are capable of?

Winston believes that hope for the overthrow of the Party lies in the proles. While he can't quite believe at first that they are capable of mounting a conscious rebellion—when he looks at them, the idea seems absurd to him—he recognizes that they have advantages Party members don't. First, there are so many of them: they constitute 85 percent of the population. Second, the Party doesn't care what they do, so they are not under constant surveillance. Winston realizes that Party members can't ever come together in groups of more than two or three, which means they can't revolt. The proles, however, can. As Winston puts it:

They needed only to rise up and shake themselves like a horse shaking off flies. If they chose they could blow the Party to pieces tomorrow morning.

At the same time, Winston understands how limited the proles are in understanding or in consciousness that the world could be different. He notes that putting his hope in the proles is an "act of faith."
Shortly before he is arrested, however, Winston suddenly realizes that the proles are human in a way Party members are not, because the proles are still allowed to live ordinary lives:

They were not loyal to a party or a country or an idea, they were loyal to one another. For the first time in his life he did not despise the proles or think of them merely as an inert force which would one day spring to life and regenerate the world. The proles had stayed human. They had not become hardened inside. They had held on to the primitive emotions which he himself had to re-learn by conscious effort.

He decides that because of their humanity they will eventually be awakened and be capable of rising up and overthrowing the Party.