“Carb” is short for a carburetor. Just like the carburetor in a vehicle, the carb on your pipe or water pipe controls air flow, and therefore the pressure, in a given space. The car hole on a pipe allows the air to come through fast at the end of your hit, mixing the smoke with air slightly to make the hit smoother and more enjoyable.
A carb cap controls the airflow on a nail or banger. It allows the air to flow directly over the oils so that they can vaporizer more efficiently. If there is no carb cap the air can flow slightly over the oils with the fresh air not coming fully into contact with the oils. When the air does not flow over the oils, but slightly above the oils, you will get weak dab hits.