Patterning your shotgun can be a time consuming task, but a very, very helpful one. Shotguns are a very diverse platform that can be used for everything from hunting, to sport shooting (skeet and trap), to home defense. A shotgun can shoot just about anything that will fit into a shot shell (as long as it is the right sized shell and the gun is designed to do so). Shot (pellets) or slugs...a shotgun can send it down range. But what is the effective range? How far away is TOO far away? These are questions that patterning a shotgun can answer!
Check out this article on why and how to pattern a shotgun. The grammer isn't great, but it does a good job at explaining the process.