ZPM or Zero Point modules are a way of dialing stargates in other galaxies as well as being able to supply a massive amount of energy to many machines.
Within the Stargate lore, a ZPM is an artificial pocket of space-time. Or to put more simply, "It's kind of like a miniature universe in a bottle." ―Rodney McKay. The device was created by the ancients or Alterans and is able to supply large amounts of energy within its crystalline structure.