It means a viewing of something before it is available to most people. The movie theater offered a sneak preview of the movie 'Les Miserables,' but most theaters will not show the film until September. Note: It does not have to refer to a movie. It can also refer to a book or even a museum exhibition.

A 'sneak preview' of a movie is a showing of the movie to a small number of select audiences, before the official, big release of the film to all movie theaters. It is used to test the reaction of people to the film or to treat a special select audience. "Audiences that were shown a sneak preview of the film in a small number of selected theaters had a positive reaction to the film." "Friends of the director were treated to a sneak preview of the film in his private studio theater."