Flow is a positive psychology concept that focuses on focus. In other words, the state of mind you are in when you have total concentration on a specific task and nothing else around you seems to matter. Total immersion. Mihaly Csikszentmihalyi, a positive psychologist primarily known as the architect of “flow,” describes flow as "being completely involved in an activity for its own sake. The ego falls away. Time flies. Every action, movement, and thought follows inevitably from the previous one, like playing jazz. Your whole being is involved, and you're using your skills to the utmost.”
Csikszentmihalyi began researching flow after he became fascinated by artists who would get so lost in their work, they would forgo food, water and even sleep.
In order to get into the state of flow, a balance of the challenge at hand and the strengths of the challenger must be met. The motivation is for no other reason than just the pure enjoyment of that certain task.
For more information watch Mihaly Czikszentmihalyi's TED talk on Flow.