Default mode network:

the default mode network (sometimes called the default network) refers to a group of interconnected brain structures that are hypothesized to be part of a functional system activated when one is awake but not involved in any specific mental exercise. Thus, the default mode network is thought to be most active during things like daydreaming, recalling memories, envisioning the future, etc. and least active during particular tasks like paying attention. Some structures that are often included in the network are the medial prefrontal cortex, posterior cingulate cortex, and the inferior parietal lobule.

Read more: Know your brain: Default mode network