Meaning

noun someone who takes more time than necessary; someone who lags behind

noun an advertisement consisting of short scenes from a motion picture that will appear in the near future

noun a large transport conveyance designed to be pulled by a truck or tractor

originFrom trail + -er. The film sense derives from the fact that previews were formerly shown after the main feature, rather than before as is usual today, that is trailing after the main feature.

Etymology from Wiktionary, CC BY-SA 3.0