A choke helps hold a shot string together, controlling the shot string’s spread when it leaves the barrel.
The shot string is the pattern of what the pellets would look like if you took a picture of the shot on its way to the target. When the pellets leave the barrel, they separate from each other due to the force of air resistance. The more the shot separates, the less effective the pellets are in taking game at longer distances, though greater separation is all to the better for closer shots.
By using a choke, you can control how much the pellets are separated at certain distances, allowing you to adjust according to game type and distance. Older shotguns have a fixed choke whereas modern shotguns feature a screw-in choke system that allows you to change between different types of chokes. This system’s main advantage is that you can use one shotgun with different chokes for different types of game and sport shooting. To change a screw-in choke, follow the manufacturer's instructions.