From a website, to give you an idea what they mean:
Flow is a "sense of effortless action [people] feel in moments that stand out as the best in their lives. Athletes refer to it as 'being in the zone,' religious mystics as being in 'ecstasy,' artists and musicians as 'aesthetic rapture.'" Importantly, "Flow tends to occur when a person's skills are fully involved in overcoming a challenge that is just about manageable," or, put another way, "when goals are clear, feedback relevant, and challenges and skills are in balance, attention becomes order and fully invested."