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A choke is a restriction in the barrel of a shotgun that presses the shot together, resulting in less of a 'scatter' and tighter groups of shot. Some shotguns have a screw in choke providing a way to change the tightness of shot groups, and others have a fixed choke, where the diameter of the barrel decreases slightly near the muzzle.

A choke on a shotgun works like a garden hose, Full Choke is like a steady stream of water, Modified Choke is half a stream of water, Improved Cylinder is like a spray of water, Cylinder Choke is a very fine mist of water coming out of the nozzle of the hose handle.* There is a choke called a Poly-Choke which attaches to the end of the shotgun barrel.* All you have to do to go from full choke to modified to improved cylinder is twist it with your hand and wrist very little to change to the Choke you want to use whether its Full, Modified or Improved cylinder. Now think of the water as BB's and you can visualize what happens, when you select different Choke settings.*

a choke is found on a shotgun

a shotgun fires a shell that can contain hundreds of pellets in one shell. as the tiny pellets are pushed near the end of the barrel, the choke tightens them closer to each other so that they travel in a smaller group