The telescreen is a primary part of the surveillance state apparatus in Oceania. Every Party member has one installed in their home. It is both a television set that gives out the ever-changing official "news" and a way for the state to monitor a person's private life. The sound of it can be "dimmed," but it can never be shut off.
Telescreens also monitor people at work and in public. As a person cannot know for sure at any given moment that they are not being watched or listened to, individuals learn to internally censor their thoughts, motions, and facial expressions to conform to the dictates of the state. Winston, for instance, keeps a look of "quiet optimism" on his face at all times lest he is being watched. In his apartment, Winston experiences a voice coming from the screen barking at him when he does not do his morning exercises with the expected gusto and precision.
Telescreens primarily surveil the Outer Party members. The proles are less regulated, which is why Julia and Winston are lulled into a false sense of security in Mr. Charrington's room. Julia and Winston are also surprised to find out that O'Brien, an Inner Party member, can turn off his telescreen at will.