Well you know what jam is..? Y'now.... sweet spread usually made with fruit and generally popular on bread, especially toast (I would mention scones but that would start WW3 on here).What’s a traffic jam?
Now imagine cars, vans, motorcycles, trucks... maybe the odd pony and trap....all held up on the roads to the point that they can't move forward. This is often achieved by the popular local authority practice of random hole placement and serves no known purpose.
This phenomenon is known as a "traffic jam" despite the fact that no jam is used in the process.