It mean that you get into what you are doing so much that you are that doing itself. Doing things with passion.
When you are completely involved in some action, that use your abilities. Time and surroundings does not matter at all.
It does not mean passivity.
How Does It Feel To Be In Flow?
(contentment)TEDx by Mihaly Czikszentmihalyi(pdf) TEDx
1. Completely involved in what we are doing – focused, concentrated.
2. A sense of ecstasy – of being outside everyday reality. Transcendence.
3. Great inner clarity – knowing what needs to be done, and how well we are doing.
4. Knowing that the activity is doable – that our skills are adequate to the task.
5. A sense of serenity – no worries about oneself, and a feeling of growing beyond the boundaries of the ego.
6. Timelessness – thoroughly focused on the present, hours seem to pass by in minutes.
7. Intrinsic motivation – whatever produces flow becomes its own reward.