Flow is a term coined by Mihaly Csikszentmihalyi as a result of studies he undertook on what gives people enjoyment. What he found is that people of all ages, nationalities and interests in life report their involvement in activities in a similar way.
In a nutshell 'flow' happens when we become so absorbed in an activity that we lose ourselves and our sense of time is altered. When we are engrossed we often have so little sense of ourselves that we do not feel happy - in fact we feel nothing. (Or indeed we may even experience physical discomfort as may be case for a mountaineer or athelete.) But afterwards we have such a strong sense of gratification that we construe the activity as enjoyable and satisfying and so such experiences contribute substantially to our feelings of happiness and well-being.
Activities which can induce flow are varied and numerous. It can come from reading a book, playing a sport or having an engrossing conversation with a friend. Commonly it is something that we find challenging. This is why many people experience flow easily in competitive sports where challenge and feedback are intrinsic to the activity. If the talents of the players are mismatched – one is a beginner chess player, for example, and the other is a Grand Master then neither is likely to experience flow. For one opponent there is no challenge and for the other it is too daunting and so not easy to lose oneself in the activity.
Another aspect of flow is that it is intrinsically rewarding and so motivating. We may be involved in the activity because it is our work, and so we are being paid to do it, yet if we are in flow with the activity we get the feeling that we would be involved even if we weren’t being paid.