what has winston discovered when he says, "the proles are human beings?" " we are not human?"

His discovery is noted in the previous sentence, as he realizes that he and Julia has become numb to the reality around them; they'd become hardened and uncaring. The Proles were still human because they had feeling; they had families, and they knew what it was to feel and act on their emotions.

"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. And in thinking this he remembered, without apparent relevance, how a few weeks ago he had seen a severed hand lying on the pavement and had kicked it into the gutter as though it had been a cabbage-stalk."