We all use devices every day that use energy - or more accurately, transfer energy from one form to another. Everything we use wastes energy - some of the energy transfers into forms that are not useful to us.
For example when driving a car, energy from burning fuel is transferred into kinetic energy of the car. However, more than half the energy is lost as heat and sound.
How effectively devices transfer energy - i.e. how much of the energy used is useful - is called its efficiency.