Mass relays' are mass transit devices scattered throughout the Milky Way, usually located within star systems. They form an enormous network allowing interstellar travel. Hailed as one of the greatest achievements of the extinct Protheans, a mass relay can transport starships instantaneously to another relay within the network, allowing journeys that would take years or even decades with only FTL drives. A Prothean data cache found on Mars led Humans to the Charon Relay, a mass relay encased in ice and orbiting Pluto, previously thought to be a moon.
Mass relays are built with two long, curved metal "arms" with a set of revolving rings in between them, containing a massive, blue-glowing Element Zero core. The relays are made of an unknown but incredibly resilient material, the same material that the Citadel is built from. One of the relays even survived a supernova's wake without being damaged. They are 'cold' objects that don't emit heat or radiation, unlike starships, making them difficult to find if their position changes. Some relays, like the Charon Relay, are 'gravitationally anchored' to planets; others appear to be out in space and are carefully tracked.
Mass relays function by creating a virtually mass-free 'corridor' of space-time between each other. This can propel a starship across enormous distances that would take centuries to traverse, even at FTL speeds. Before a vessel can travel, the relay must be given the amount of mass to transit by the ship's pilot before it is moved into the approach corridor. In Mass Effect: Revelation, when a relay was activated, it moved to align itself with the corresponding relay before propelling the ship across space.
Many Mass Relays are currently dormant for unknown reasons, though they can be easily reactivated. Humanity's 'home' relay, the Charon Relay, had become encased in ice, but scientists were able to reactivate it using data recovered from the Prothean ruins on Mars.
There are two kinds of Mass Relay, primary and secondary. Primary relays can send a ship hundreds of light years but only link to one other relay, its 'partner'. Secondary relays can link to any other relay over shorter distances. After the Rachni Wars, space faring species won't open a primary relay without knowing where it links to, in case they run into another powerful and hostile species like the Rachni. This caused a rift when the Turians found Human pioneers, ignorant of this Citadel Council prohibition, trying to open any Mass Relay they could find while exploring the relay network, eventually leading to the First Contact War.
A piece of Prothean artwork depicting a Mass Relay, the Relay Monument, can be found on the Citadel Presidium. It has been interpreted as either a symbol of Prothean vanity, expressing the relays as their means to build a galaxy-wide empire, or possibly as a symbol of galactic unity, which the relay network also embodies. Tali'Zorah nar Rayya claims that after their long voyage through the Galaxy, the Quarians have also come to appreciate the aesthetic value of the Mass Relays.