In psychology, “flow” is simply:
The mental state of operation in which a person in an activity is fully immersed in a feeling of energized focus, full involvement, and success in the process of the activity. — From Wikipedia
We experience flow as a result from concentration. Once your mind zones in on one specific thing (because remember: your mind cannot multitask), you enter a trance where you do not think before you do something, you just do.
And that’s what flow is – doing without thinking.