Flow is when your body and mind become fully immersed in whatever activity you are participating in. Your mind doesn't wander and your perception of time changes.
In order to maintain a state of flow, a person must not feel threatened in any way. Distractions should be kept to a minimum.
Flow requires that you have a clear and reachable goal in mind.
It also requires that you both receive feedback for your actions in addition to having control over them.
What triggers a flow state in one person may not have the same effect in another.