Cowl flaps, in aviation, are just what their name implies - flaps in the cowling which covers the engine or engines (generally piston engines, but not always). They're typically used on small piston aircraft with more powerful engines. In some cases, those engines need additional cooling at high power settings or at very low speeds - but the pathways required for that airflow are enough to cause drag on the airplane which is undesirable. In those cases, the airplane may have cowl flaps. A control (lever, usually) in the cockpit will allow the pilot to open and close the flaps in certain conditions, generally as part of a checklist. For example, in the Cessna 182 Skylane, the cowl flaps are opened prior to starting the engine on the ground, and are left open while taxiing, during takeoff and during climbout. They are closed when the airplane is trimmed for cruise and the RPMs and power are brought down. They are opened in enroute climbs as well, when additional power is required.