A totalitarian state is that which is run by one party, or more often, by one person, with little to no individual freedoms or rights. The people are often referred to in the collective (for example, "Peoples Republic of China") and the importance of the society, in the form of the State, is continually emphasized.
In Darkness at Noon, Koestler reveals the ugly inner workings of a totalitarian state: that of Soviet Russia in th 1930s. The leader of the Soviet Union at that time was the ruthless dictator Josef Stalin, who ran the USSR from 1924 - 1953, and in the process, murdered an estimated 20 million of his own countrymen.
Koestler also does a masterful job of portraying the totalitarian state as controlling, or attempting to control, individual thought and ideology. The control of collective and individual thought, or at least, its expression, is central to the continued authority of the Soviet State, or any totalitarian government, for that matter.