The knob on the side of your fireplace is likely a damper control. The damper is a metal plate or flap that is used to control the amount of airflow through the chimney. By adjusting the damper using the knob, you can regulate the intensity of the fire and control the amount of heat released into the room. Opening the damper increases the airflow and allows more heat to escape through the chimney, while closing restricts airflow and keeps more heat inside the room.