Mihaly Csikszentmihalyi, the founder of flow describes it as "a state of mind when consciousness is harmoniously ordered".
It is an experience where the mind and body work together effortlessly. When one is totally absorbed in what one is doing. Where one is challenged but perceives they have the skills to match. It is an intense experience and requires all of one's available attention, hence there is no attention left over to worry about irrelevant things.
It is a pleasurable experience and intrinsically rewarding. And it can be addictive.
The flow mindset has nine dimensions with three of these forming the foundation; the challenge skills balance, clear goals and unambiguous feedback. The remaining six are characteristics of the flow experience; focused attention (effortless), action-awareness merging, sense of control, loss of self consciousness, transformation of time and autotelic experience.
The model below shows more graphically how the flow experience is constructed.