Proles are neither members of the Inner Party nor Outer Party and enjoy relative freedom compared to those like Winston Smith who are under constant surveillance. However, they are also kept poor and ignorant and do not truly understand their own oppression. One of the Party's slogans captures their place in society quite well: "Proles and animals are free." Thus, the Party equates this group constituting around 85% of the population to having the worth of animals in this society. The proles are not generally required to keep telescreens in their homes. They are allowed liberal sexual freedom. They enjoy football and beer and fight with their neighbors. And they aspire for little else.

The Party also uses them as minions to keep proper surveillance on the happenings of society and to root out any signs of traitors. Consider this comment from book 1, chapter 5:

That's a first-rate training they give them in the Spies nowadays – better than in my day, even. What d'you think's the latest thing they’ve served them out with? Ear trumpets for listening through keyholes! My little girl brought one home the other night – tried it out on our sitting-room door, and reckoned she could hear twice as much as with her ear to the hole.

Even children are equipped with technology to aid in their spying on their parents—and others, should the opportunity present itself. However, the proles don't even seem to consider the implications of such uses of technology, mindlessly complying with a sort of vague fascination with the tactics of the Party.

Though their numbers give them a large advantage should they decide to revolt, the Party doesn't consider the Proles a threat. Winston realizes early on that if their society is going to change, the revolution must come from those with the greatest advantage in number:

But the proles, if only they could somehow become conscious of their own strength, would have no need to conspire. (book 1, chapter 7)

But because they are given just enough to eke out a survival and just enough to keep them superficially entertained, the Proles remain a mindless, complacent mass who generally don't question their society's sense of order. O'Brien himself captures the sentiments of the Party's worry about this large segment of their population:

The secret accumulation of knowledge – a gradual spread of enlightenment – ultimately a proletarian rebellion – the overthrow of the Party. You foresaw yourself that that was what it would say. It is all nonsense. The proletarians will never revolt, not in a thousand years or a million. They cannot. I do not have to tell you the reason: you know it already. (book 3, chapter 3)

Thus, their society's structure continues because those with the greatest power to initiate change don't have the forethought to question their world or to believe that things could possibly be different.

The proles are the ignorant masses that make up the majority of Oceania's population. The Party oppresses the proles through economic disparity, propaganda, manufactured hysteria, and manipulation. Even though proles account for eighty-five percent of the population, they do not recognize that they are being controlled and manipulated by Big Brother. The proles are significant to Winston Smith, who hopes that one day they will revolt against the Party and put an end to the totalitarian regime. Winston writes in his diary, "If there is hope. . . it lies in the proles" (Orwell, 89). Winston even interviews an older prole man in an attempt to compare the past with the current dystopian society to disprove the Party's historical accounts before the Revolution. Unfortunately, the man cannot express a sensible thought and does not give Winston the information he is seeking. Winston continually frequents the prole section of town and even rents an apartment above Charrington's antique shop. When Winston looks out of his window in the prole section, he sees a prole woman singing while doing laundry. This prole woman becomes an important symbol for Winston, and she represents hope in the future. Orwell also uses the proles to symbolize the uninformed masses that passively accept authoritarian regimes. Unfortunately, Winston's hope in the proles does not come to fruition by the conclusion of the story.

The proles are the great mass of people who are not members of the Party in Oceania, and yet who make up the bulk of the population. The Party more or less dismisses them as of no importance. Winston harbors the romantic illusion that the proles will revolt and take down the government. In "The Book," however, we learn they are kept too poor and ignorant to mount any kind of ideologically coherent campaign to overthrow the oligarchy in power.

The proles, however, are more important to the plot than simply as representatives of the downtrodden masses in this dystopic world. In the world of the proles, Winston finds remnants of his dimly remembered childhood before the Party gained control. He buys a diary and a paperweight with a piece of coral inside it from the proles (or so he believes). He and Julia rent the room above Mr. Charrington's shop and, for a brief time, recreate some semblance of a normal, old-fashioned love affair.

But the Party's secret police also use the proles as a cover to nab transgressors like Julia and Winston. We learn that Mr. Charrington is not a prole but a member of the secret police. While we are never told so definitively, Orwell strongly hints that the washerwoman endlessly hanging diapers in the courtyard below their window at Mr. Charrington's is also a spy: Winston has to creatively invent more and more grandchildren for her to justify the amount of laundry she hangs. Proles thus become both romanticized symbols of the past onto which Winston projects his fantasies, ideal "covers" for a spying secret police, showing how far the network of surveillance has spread, and a mass of suffering people living in miserable homes and enduring rocket attacks.

The proles are the working people -- the ones who provide all the more physical and manual labor that needs to be done in the society.  For example, they serve the food at the cafeteria at Winston's workplace.  They make up about 85% of the society.

To me, the main importance of the proles to the story is that Winston thinks that they are the society's hope for the future.  He seems to think that they are more closely connected to what people used to be like -- they have more "human" lives than they people in the Party who are constantly monitored.  He thinks that they are much more likely also to be able to get together to start a rebellion because they are not monitored.