I see people talking about the default mode network in the brain nowadays in relation to mindfulness. As far as I understand it, normally when you're focused on an external task, the default mode network is inactive. When you're not focused on any external task, the default mode network activates and subjectively this feels like daydreaming or mind-wandering.

Mindfulness meditation is a technique where you learn to keep the default mode network inactive while not engaged with any external activity. The resulting subjective state feels quiet (as opposed to constantly occupied with stray thoughts) and somewhat like Csikszentmihalyi's flow.