Mass Relay

Mass Relays are a network of devices in space usually located within a star system that allows interstellar travel. The Mass Relay system transports starships instantaneously to another relay anywhere within the network. There are collections of nodes that allow the networking of the relays to operate and pass on information to the other relays across the galaxy. The relay found within the Solar System was located within Charon, Pluto's moon underneath massive amounts of ice. Humans found it from the information located within the cache found on Mars.

The Mass 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.

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, spacefaring 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.

The Relays, despite the common belief amongst the races in Mass Effect, were not created by the Protheans but by the Reapers. As made clear by Sovereign, the Protheans were merely one of many alien races to find the Relays and the Citadel and take advantage of them. When the Reapers wiped out the Protheans, the Asari were the next race to find the Relays, thousands of years later. According to Sovereign, using the Relays means that organic civilisations evolve according to the Reapers' wishes, but it refused to explain further.

However, the Protheans did have a keen interest in the Relays, and managed to crack the secret behind their creation. This enabled them to build the Conduit before being destroyed by the Reapers.