At the very outset of 1984, Orwell establishes the setting as one in which all citizens of Oceania are continuously being spied on. When Winston sits down to write his diary, he can only do this because of a fluke in the construction of his flat, where a small nook is hidden from the telescreen. Obviously it's a massive form of control if everything a person does, even in their own home, is being watched by a brutal regime in which the slightest provocation is punished by torture or death.
Similarly, the only way Winston can read the note saying "I love you" that Julia has surreptitiously passed to him is by making it appear part of the bundle of documents he's planning to falsify when he sits down at the "speak-write" in his work cubicle. He has considered unfolding and reading it in the men's room but knows that even in the stalls (or especially there) he would be closely surveilled. Under these conditions everyone, one would think, is being kept in a continuous state of anxiety, to the point where the Party is limiting or controlling people's actions and practically making them into puppets.
The telescreens are also constantly transmitting as well as receiving—that is, transmitting not just video but audio as well, constant sound. The unending sound would, we would imagine, drive people crazy. Only the Inner Party members like O'Brien, as he himself tells Winston and Julia, are able to turn off the telescreen.
Yet we're told that "most of the proles did not even have telescreens in their homes." Why would this be the case? There are, arguably, two reasons. First, Orwell is probably emphasizing the primitive conditions under which the working class live which, like everything else in the dystopia, are an exaggeration or extension into the future of the way things already are. The unequal conditions of an industrialized world were examined repeatedly by Orwell in his earlier books such as Down and Out in Paris and London, Keep the Aspidistra Flying, and The Road to Wigan Pier. But the second reason is that the Party simply doesn't bother with the proles—they are beyond the pale, not even thought of as human beings. Hence, Winston and Julia think they have nothing to fear by being together in the room above Mr Charrington's shop. It turns out, of course, that they could not have been more mistaken, since the whole scenario is a set-up and Charrington is an agent of the Thought Police.