In Positive Psychology, a flow state, also known colloquially as being in the zone, is the mental state in which a person performing an activity is fully immersed in a feeling of energized focus, full involvement, and enjoyment in the process of the activity. In essence, flow is characterized by the complete absorption in what one does, and a resulting transformation in one’s sense of time.
(Wikipedia: https://en.wikipedia.org/wiki/Flow_(psychology))
The states in which you have so much of your consciousness engaged in doing something that it seems like you are on autopilot and you lose sense of everything including even your awareness of time, your surroundings, and yourself. Thinking about how to do something interferes with and interrupts this process, which is why every discipline involving any kind of skill (physical, mental, or even creative) will emphasize some kind of drill/practice in order to build the necessary procedural memory to engage a flow state with minimal mental priming.