The choke is a small hole about a quarter-inch wide on the side of
pipes and
bongs usually used for smoking
marijuana. Also called a
carb, you hold your finger over the choke while inhaling, permitting the
chamber to fill with smoke.
With a pipe, when the chamber is full you may remove your finger, permitting a rush of air into the lungs. Inexperienced smokers may partly cover the hole on a pipe to permit a mixture of air and smoke to be inhaled instead, resulting in a milder, less painful hit.
With most bongs, the smoker holds their finger over the choke until the chamber fills with a sufficient amount of smoke, then, generally they remove the finger, letting air rush into the chamber and pushing all the smoke into their lungs.
Essentially a choke is a method of allowing all smoke to be removed from inside of the device, minimizing waste.