The conservation of energy law states that energy can't be created or destroyed. Energy can only be transformed from one form to another, and the total amount remains the same throughout the entire process.
One example of this principle is the way a car changes the chemical energy of burning gasoline into the mechanical energy used inside the engine, which makes the vehicle accelerate. Another example is how the electrical energy created by a windmill is transformed from the mechanical energy of the turning blades. Since this conversion process isn't always perfect, sometimes a significant amount of energy is wasted when some energy is turned into wasted heat.