The configuration of the top and bottom vents matters more than you think.
Unlike the fixed vents that run across the rear walls of gas grils, the vents on charcoal grills, which are located on the top of the grill lid and at the base of the kettle, are adjustable and give the cook the power to control the strength of the charcoal fire as well as the flow of smoke.
Their operation and function are pretty simple: When the grill lid is in place and the top and bottom vents are fully open, air flows through the grill and allows the fire to burn at the maximum heat level. When both vents are fully closed, the air supply is cut off and the fire will eventually die out. You can modulate the strength of the fire by adjusting the vents accordingly.
The position of the top vent on the lid also directs the flow of heat and smoke, which rises from the coals and is drawn out through the lid vent. You can position the vent minimize or maximize heat and smoke contact with the food you're grilling.
The bottom line: Adjusting the top and bottom vents on a charcoal grill allows the cook to control the heat and the flow of smoke inside the grill.