Refer to figure.
Many light aircraft piston engines are air-cooled to save weight and complexity. Cowl flaps are movable flaps in front of an engine that can be opened and closed.
When they are open, they provide cooling air to the grills around the cylinders, and this improves cooling, so is useful during take-off and climb, for instance.
When they are closed, they reduce the drag of the aircraft and also reduce the airflow (there is still some airflow) over the cylinder grills, to reduce the chance of shock cooling, such as in the descent.

They directly cool the cylinder heads, which is the hottest part of the engine, and must be controlled to avoid excessive engine wear and detonation. Cowl flaps are one way to control the CHT (Cylinder Head Temperatures), along with mixture control and throttle.