Using the correct choke tube in a shotgun can be the difference between a successful shot and a miss. A shotgun choke is simply a tapered constriction of the gun's bore at the muzzle end. The constriction of the choke determines the shotguns pattern. With screw in chokes you can very easily change the degree of constriction by simply changing the choke. This allows the use of one shotgun for almost all wingshooting activities.
Patterns are normally expressed with a name such as improved cylinder or as a percentage such as 50%. When expressed as a percentage it is the percentage of the pellets that are in a 30” circle shot from 40 yards away. The exceptions to this are cylinder bore and skeet 1 chokes in all gauges and .410 bore shotguns in all choke sizes. These are measured at a distance of 25 yards.
Selecting the right choke is not necessarily a simple task. The two largest factors in deciding which choke to use is the distance away from your intended target and what your intended target is. There are all sorts of guides and charts that can help you get an idea of what choke to use, but pattern performance can vary by manufacturer of the choke tubes and especially with different loads. The only true way to know how your gun, choke tube, and ammunition performs is to pattern it