What does the idiom 'be on the road' mean?

To be on the road means to be traveling. You often hear this used in reference to musical groups or other entertainers. They are "on the road," meaning they are traveling from place to place to put on their shows. You would say "I'm on the road," if you were traveling to or from work and someone called on your cellphone, for example. You could also say "I'm going to be on the road next week." if you were planning a trip of some sort.