"The Chosen One" is a title canonically given to Anakin Skywalker of the Star Wars continuum, and to Harry Potter of the Potterverse.
Generally, the bearer of this title is destined for individual greatness, and who (presuming they succeed in their mission) are considered the saviour of their country / world / universe / whatever. It is therefore no surprise that many Mary Sues try to call themselves such, and that it is a fairly common charge.