A perpetual motion machine is a fictitious construction that releases more energy than it uses. So practically a device that generates an infinite amount of energy. Again and again there are ideas how to implement this, but so far everyone has failed because of physics.
Because the law of conservation of energy applies:
In a closed system, energy can neither be generated nor destroyed. Energy can only be transformed. For example, a wind turbine converts the kinetic energy of the wind into electrical energy.