In the article Get in the Zone: The Psychology of Video Game Design
What does Millar mean when he refers to players being in a “state of flow?”

 involved in an activity for its own sake. The ego falls away. or the mental state in which a person performing some activity is fully immersed in a feeling of energized focus, full involvement, and enjoyment in the process of the activity. Wikipedia

Explanation: According to positive psychologist Mihály Csíkszentmihályi, what you are experiencing at that moment is known as flow, a state of complete immersion in an activity. He describes the mental state of flow as "being completely involved in an activity for its own sake. The ego falls away.

bildungsroman is a german word for novel covering youth to adulthood which describes a coming-of-age story

i'm a frog i'm a frog i read about frogs i breath thee frogs the frogs are great but not me