The flow is a concept of the hungarian Milhály Csikszentmihályi. He interviewed many people about their jobs and everyday activities and ask them to write down about ten times a day what they are doing and how they are feeling. Some of the timing for the questions was random.
If you’ve ever felt like time is just flying by, you were probably in the state of flow. Flow is a highly focused state in which you focus on one topic and ignore everything else. Your task can be quite simple ie. there was a worker who had to put together parts as soon as possible, so the worker challenged himself to be as quick and efficient as possible. This way he was focused, he was in the flow and the enjoyed what he was doing.