What does a choke do?

Answer 1

The choke increases the fuel to air ratio in a cold start situation by restricting air flow a cold engine requires more fuel to air ratio to run than a warm or at operaring temperature engine.

Answer 2

The question may, of course, be referring to the choke of the barrel of a shotgun. Most shotgun barrels are narrower to a greater or lesser degree at the muzzle than at the breech. This is in order to make the shot more focussed and less spread out. Double barrelled guns will probably have each barrel with a slightly different choke. This enables you to choose the best shot pattern for the distance and the type of quarry.