means A trailer is a vehicle towed behind another, a haulable home or cargo box on wheels, or — confusingly — a short promotional preview of a film or show.

from From the verb 'trail,' itself from Old French 'trailler' (to tow, to drag along), with roots reaching back toward Latin 'tragula,' a kind of dragnet. A trailer was literally a thing that trails behind — so the wheeled sense is the honest, original one. The movie sense is the odd cousin: early film previews were tacked onto the END of a reel, trailing after the feature, which is exactly where they made the least sense and were promptly moved to the front, keeping the now-backwards name.