So I take it the chick is the new Jedi apprentice?
Super spoilers ahead.
Rey is a scavenger on the desert world Jakku who is possibly actually a descendant of Luke or Leia but doesn't know it herself. The line that Luke says in the trailer can only possibly be said to her going by the scenes that we know thanks to MakingStarWars.net, which would mean she is almost certainly related to one of them, although how exactly this works out is kind of confused right now. We know that Luke set up a Jedi academy between Episodes VI and VII but that it was massacred Order 66 style with few if any survivors. Rey apparently may be one of them (or the only one) and somehow escaped/was hidden away as a child. The end of the movie is Rey finding Luke in his exile and returning to him his lightsaber, accepting her destiny as his padawan after running away from it for the course of the film.