The Chosen Ones were human children whom Fairies would protect and avenge if harm came to them. Eventually, the Fairies would claim these children and take them to their "Lost Lands". Once there, the children would become Fairies themselves. The Fairies required these children to continue their race, despite being immortal (or near-immortal).
In 1909, after a night of heavy drinking among his unit, some of Jack Harkness's soldiers accidentally ran over and killed a Chosen One in Lahore. In retaliation, the Fairies killed all his men, sparing only Jack.
In 2007, Torchwood Three allowed the Fairies to claim Jasmine Pierce as their Chosen One. Jack permitted this on grounds that the Fairies would cause immense destruction beyond Torchwood's control if they were deprived of the newest addition to their ranks. Before her departure to the "Lost Lands", Jasmine said "Thank you" to Jack in a Fairy-like voice. (TV: Small Worlds)
Behind the scenes
This connection between human children and Fairies seems to pay homage to the writings of Welsh author Arthur Machen.[source needed] He believed in fairy folk, and that these ancient creatures would claim one human life every time they emerged to replenish their ranks.