Think about the last time you drove into a major city. Chances are you traveled on a highway that allowed many cars to travel quickly to their general destination.
If you imagine our highway system as transmission lines and cars as electrons, you have a good analogy to describe how our grid works.
Like highways, transmission lines carry bulk energy quickly across long distances and across state lines. But again, like highways, when transmission lines are overburdened they become less efficient and reliable.