Shotguns generally shoot small pellets, called shot. The muzzle end of the shotgun barrel has a choke, which is used to control the spread of the shot. The choke in a shotgun is very much like the nozzle on a garden hose. You can make the water shoot in a narrow stream, or you can make the water shoot in a wide stream.
Some shotguns have fixed chokes. That means there is only one type of choke in the barrel. To change the choke, you must change the barrel. Other types of shotguns have adjustable chokes, or screw-in, interchangeable choke tubes.