The Sight is the ability that separates Shades from ordinary humans, allowing them to see the ghosts of the dead who are still lingering in this world. It is a power that develops from a Near Death Experience, when the person on the brink of death experiences a feeling of nothingness, where a white light envelops them and from there they are brought back to the living world as a slightly changed person. However, further investigation by Jane Quaint reveals that a person can only develop the sight if they have the gene that allows them to develop it, passed down from their ancestors.
It is implied to be a rare ability. During Rory and Boo's conversation with ghost Alistair, he claims that he has only ever encountered one or two people with the Sight in over two decades of lingering at Wexford.