A Chosen One is a Human child that Fairies protect and avenge until the time they claim that child and turn into into one of their own kind. The Fairies require these children in order to continue their race's existence. The Fairies protect the child, sometimes by killing the attacker.
In 1909, one of Jack Harkness's soldiers accidentally killed a Chosen One in Lahore. In retaliation, the Fairies killed all his men, sparing only Jack.
In 2007, Torchwood Three was unable to stop the Fairies from claiming Jasmine Pierce as their Chosen One. (TW: Small Worlds)