Before the days of movie preview trailers films were promoted with glass advertising slides (and so were products, even in the silent days, they had commercials). 

Often quite colorful and artistic, the slides were designed to get the audience into the theater for the next new feature (films generally ran a few days at best).  That was not the case with Robin Hood, however, it ran for quite some time in Hollywood.  So long, in fact,  that the streetcar conductors would call "all out for Robin Hood" rather than the street name of the scheduled stop.

The use of slides continued up to the early 1940s.  Many survive and they are quite collectible today.