a power source is a device that supplies some form of energy, though it's most used to describe sources of electric power, like an electrical generator or a battery.
Most power sources work by converting one type of energy, like chemical, nuclear, heat or motion energy, in to the form of energy desired. The most important energy source we have is the sun, since only nuclear energy is totaly independant(Actually the sun is a nuclear reactor, since it gets it's energy from a hydrogen fusion process, so you could say all our energy come from nuclear reactions.)