The film: Kevin Costner plays an Iowa farmer who, prompted by a ghostly voice given to whispering nonspecific instructions, decides to build a baseball field on which his estranged father and the disgraced 1919 "Black Sox" can throw the ball around. (Admittedly, this is one of those movies that are much better than the premise suggests.)
The line: "If you build it, he will come."
The setup: Spoken by "the voice" to Ray Kinsella (Costner) as he wanders through his cornfields at dusk. The he in the quote refers to Ray's dead father — and to a lesser extent, to White Sox slugger "Shoeless" Joe Jackson. It's commonly misquoted as "If you build it, they will come."