This expression refers to pervasive surveillance by the government. It comes from the book Nineteen Eighty-four written by George Orwell which was first published in 1949. Orwell wrote the book in 1948 and he just reversed the 4 and 8 to refer to the year 1984, which then seemed far in the future. The story takes place in 1984. At that time the world is divided into three major countries and always two of them are at war with the other. From time to time, countries that were allies become enemies and vice versa. The governments of these three countries are similar and their goals are the same: keep the people under control by always waging war, through surveillance, propaganda, and the frequent rewriting of history.
The government is similar to the Communist Party in China or North Korea. At the top there is a Supreme Leader who in the novel is called Big Brother. Below him is the Inner Party which is a select group of powerful subleaders. Below it is the Outer Party who are the lower bureaucrats. There is another group called the Proles who basically perform menial tasks and are below the Outer Party members.
The government is able to constantly watch everybody through TV screens placed everywhere and in all homes. People must believe what the Party dictates and anyone who is suspected of not believing is arrested, tortured, and ultimately killed.
An excellent movie that faithfully follows the book was made in 1984 and starred Richard Burton and John Hurt. I strongly recommend it.
Probably if you read the book or watch the video, you will see parallels between how countries are ruled in the story and what actual countries are doing today, especially North Korea.
 (especially an unwelcome influence or physical effect) spreading widely throughout a an area or group of people;広がる
 Close observation, especially done unobtrusively [目立てないように] by a government agency or group for the purpose of detecting unauthorized behavior; 監視