Efficiency is how much much energy you get out for how much energy you put in. Generally, there is friction between objects, have you ever gotten rug burn? That's friction. Friction takes motion and turns it into heat, that's why rubbing your hands together makes them warm. The more complex a machine, and the more parts it has, the more opportunities friction has to turn the energy you're putting in into heat. When friction makes heat, it is slowing down an object and using the objects energy. So the more friction does, the less efficient your machine is.
