A mass relay is one of the largest examples of mass effect technology. It uses an enormous mass effect field to launch starships across interstellar distances, enabling journeys of thousands of light-years in a matter of seconds.
Mass relays come in two types. Primary relays can only launch ships to a single destination. Secondary relays have a shorter range, but they can propel a ship to one of several preset destinations. In either case, the destination must be another mass relay. Objects transported by mass relays commonly have "drift" from the target relay of up to hundreds of thousands of kilometers.
Mass relays are protected by kinetic barriers of such strength that no known mass accelerator weapon can damage them. It took a collision with a major asteroid to destroy a mass relay, and that event released enough energy to render all planets in the system uninhabitable.