Mass Relays are huge travel devices that allow starships to travel to different systems in matter of seconds. They are considered as the Prothean's greatest creation. It consists of two curved poles made of an unknown material with a giant ball of Element Zero in the middle. When a Starship gets close, the Mass Relay drags it in and fires it out of the system like a gun.
The Mass Relay closest to Earth is called the Charon Relay. After the Humans discovered a Prothean data cache on Mars in 2148, they found traces of Element Zero on the cache. This led to a huge advance in technology and allowed the Humans to travel further into the Sol system. They discovered that a moon surrounding Pluto called Charon was actually a Mass Relay frozen in ice.
It's popular belief that the Protheans created the Mass Relays. However, later in the game this will be proven wrong. It was actually the Reapers who created the Mass Relays and the Citadel. They created them as bait to begin a cycle of galactic genocide. The Reapers wanted other species to discover the Mass Relays and use the technology to expand across the universe. Then when the Galaxy was reaching it's peak in technology, the Reapers would appear and destroy all traces of their existence and start the cycle again.