When subjected to an electrical current, the rare material dubbed element zero, or "eezo", emits a dark energy field that raises or lowers the mass of all objects within it. This "mass effect" is used in countless ways, from generating artificial gravity to manufacturing high-strength construction materials. It is most prominently used to enable faster-than-light space travel.
Eezo is generated when solid matter, such as a planet, is affected by the energy of a star going supernova. The material is common in the asteroid debris that orbit neutron stars and pulsars. These are dangerous places to mine, requiring extensive use of robotics, telepresence, and shielding to survive the incredible radiation from the dead star. Only a few major corporations can afford the set-up costs required to work these primary sources.
Humanity discovered refined element zero at the Prothean research station on Mars, allowing them to create mass effect fields and develop FTL travel.