A barker is a person whose job involves advertising a show or a circus, encouraging passing pedestrians to buy a ticket. A traveling carnival might employ a barker to describe its attractions.
A barker's job includes detailing the highlights of the show or fair he's trying to sell. An old-fashioned sideshow carnival barker might call to passers-by, describing the bearded lady or human pincushion inside. In the 15th century, a barker was a "noisy fellow," which became a "loud assistant or salesman" by the late 17th century. It comes from bark, from its Old English root beorcan, "to bark."