Entertaiment is a way to have fun. It is a form of entertainment, wich can give you a good feeling, and it stimulates the brain to realease seratonin and dopamine. A lot of entertainment hits on the points that our minds was shaped to react deeply to, such as murders and backstabbing in social groups. This is because our ancestors needed to be aware of the social world in order to survive, and the brain was shaped to understand the social world deeply.
Some forms of entertaiment are reading books for pleasure, playing sports and video games, visiting museums and galleries, and creating or appreciating visual art.