Some of the power developed by the cylinders is lost in overcoming the friction of the engine components. Some power is also used to drive the engine accessories such as the pumps. The Brake Power of the engine is the useful power available.
Thus,
Brake Power = Indicated Power - Frictional Power
Friction Power
The power used to overcome the frictional resistance and make the engine rotate is called the Friction Power.
An engine which runs without load is developing only Friction Power.