Film trailers are named after the fact that, despite being used to promote upcoming works as a preview, they were first shown after - trailing - the main feature in the cinema.
The practice dates back to 1913 when Nils Granlund, advertising manager for Marcus Loew’s chain of cinemas, decided to produce a short promotional film for an upcoming Broadway musical and screen it after the main film. The following year, Granlund would debut advertising a Charlie Chaplin film in a similar way.
Variations on the format were introduced - inclusions of cartoon reels (such as the 1930 and 40s Looney Tunes and Merrie Melodies cartoons), newsreels and other material after the main feature, as audiences had a tendency to leave once the film was otherwise over.
In the modern era, what were formerly known as trailers often now get screened ahead of the feature, and either mid-credits teasers and interludes, or sometimes blooper reels, are shown at the conclusion of the film.