The post office makes sure pieces of mail and packages are delivered to millions around the United States. The City Carrier, or CCA, is an assistant for the regular carriers that have an assigned route. Coming in a carrying a route for a particular station and carrier and making sure that all of the mail on assigned route is correctly distributed.
However, you do not have your own route that you can learn until you make regular, which can take 6 month to years.
Until then, you bounce from station to station picking up routes that regular carriers cant carry due to vacation injury, or advancement.