The telescreens were television and security camera-like devices used by the dictator of Oceania (one of several huge land masses) to prevent anyone in his realm from forming conspiracies with others against the government. These large screens (or video-phones, since Winston's exercise instructor speaks to him in the mornings) were so sensitive that they could detect your heartbeat! A few privileged people could turn off their telescreens with the understanding they could only be off for 30 minutes or less. No one ever knew how many screens were monitored at any one time or how.
Of course, such technology is impossible, but in the grip of the plot, it is quite believable and scary, because people were afraid that the telescreens could even read your thoughts, which were constantly being molded by the outlawing of most words with shades of meaning. This is in keeping with the real-life Sapir-Whorf hypothesis that "language shapes thinking" or "language anchors thought."
On the telescreen was the dictator’s assumed head and shoulders. He was “Big Brother” (aka “B-B”), the ultimate propagandist, who resembled the Soviet dictator Joseph Stalin. The phrase “Big Brother is watching you” was constantly repeated on the screens, along with cheap music, false military news reports, patriotic anthem shorts, and a "Two-Minute Hate" film. All of which is highly evocative of the mass-communication techniques of Hitler's Nazi Germany.
The book’s protagonist, Winston, is able to keep a secret diary, because he has a nook in his tiny apartment that is just out of the reach of its telescreen.