What's a Rerun?

There is a scene in Back to the Future where Marty McFly (Michael J Fox) is back in 1950 (from 1980) and sees a show he's seen before. He calls it a rerun. One of the kids asks, "What's a rerun?", because it's a live broadcast and reruns don't exist yet.

I saw this scene recently, so I came home work and asked Hudsons, "Do you know what a rerun is?". She replied she didn't. We've come full circle.