A totalitarian government is a government that is usually under the control of a single political party that tries to control every single aspect of everyday life. In a totalitarian government, there is no opposition, no "second" opinion. You're either pro-government or declared an enemy of the state. Everything in a totalitarian government is controlled from what is taught in schools to what you buy at the supermarket. Propaganda is everywhere and democracy is usually suppressed.
In video games you're often tasked to taking down a totalitarian government by acts of sabotage.