Suppose you're in charge of building a bridge across the river going through your town. What's the first thing you might think to do? Well, the simplest idea is to just stick some big concrete posts all the way to the bottom of the river, and lay down segments of road on top of them, like this.
But each post (called a pylon) costs a lot of money to put down. Moreover, they're so close together, some of the bigger boats on your river can't fit through.
You might then think of just increasing the distance between the posts. But this has its own problems.
You need some way of supporting these longer segments. So instead of laying down the road, you hang it up!! You make the few pylons that you have very tall, then drape some strong cable between them, and hang the road from the cables.
Here's a picture of them hanging the road during the construction of the Golden Gate Bridge.
