Whenever energy is transferred from one form to another, some of that energy is lost as heat. This is due to the second law of thermodynamics, which states that no energy transfer is completely efficient. In any energy conversion process, a portion of the energy is always dissipated as heat, which is often unusable and considered a loss.
For example, when you turn on a light bulb, electrical energy is converted into light and heat. While the light is useful, the heat generated is a byproduct and is essentially lost energy. Similarly, in a car engine, chemical energy from fuel is converted into mechanical energy to move the car, but a significant amount of energy is lost as heat through the exhaust and engine components.
This phenomenon is why energy efficiency is such an important consideration in engineering and technology. By minimizing energy losses, we can make better use of the energy we have and reduce waste.