"The rush hour" is the time of day when the majority of people are travelling to and from work. There is a morning rush hour and an afternoon or evening rush hour. Of course, this period of time doesn't literally last exactly an hour. It might be a 2-3 hour period, like from 8:00-10:00 in the morning and from 5:00-8:00 in the evening.

In the example above, notice that "rush hour" is being used as an adjective to describe the "traffic".