A cul de sac is a dead-end street, particularly one with a circle for turning around at the end. Kids who live on a cul de sac have a safe place to ride their bikes without a lot of busy traffic.
Cul de sacs are designed to slow and limit the number of cars that pass through residential areas — because a cul de sac doesn't connect to another road, people can't use them as shortcuts, and they're typically mostly traveled by people who live there. The French cul de sac was originally an anatomical term meaning "vessel or tube with only one opening." It literally means "bottom of a sack," from the Latin culus, "bottom."