Emergence is the appearance of system-level patterns or properties that no single part has and that arise from the interactions among parts. A traffic jam is a pattern of the road system, not a property of any one car.
Example
In a simple traffic model, adding occasional random braking to cars that otherwise drive smoothly produces jams that travel backward along the road, even though no driver intends one. The pattern exists only at the level of the whole road.
See Emergence Explained: Weak vs Strong Emergence in Complex Systems.