"very few of us survived, and those who did, owe their lives to the Traveler ... Legends say The Traveler sacrificed itself to save us. The mysterious sphere still hangs where it made its last stand, low above the Earth, keeping silent watch."
The Traveler is a massive NPC in Destiny
The Traveler is an interstellar being discovery by humanity on the surface of Mars. It is something from well beyond the Solar System humanity resides it. Exactly what the Traveler is unknown and there is a lot speculation to that point. It could be a hyper intelligent being, a large vessel or an Artificial Intelligence.
Its discovery on the surface of Mars led to a Golden Age for humanity. With its help, humanity began to spread out to the stars. This could have been what drew its foe, The Darkness to Earth. The Traveler made a final stand against the Darkness over Earth. It has hung silent above The City since then. It is as though the Traveler is keeping a silent vigil over the remnants of humanity.
The Traveler is the source of the powers of the Guardians. It seems to have its Ghosts seek out their partners from the ancient dead.