A trailer is a wheeled vehicle that can't move on its own — it needs to be pulled by a car, truck, or other vehicle. Some bike-riding parents tow their small children behind them in bicycle trailers.
The trailer you might put a boat on, hitching it to your truck, trails behind the vehicle that's pulling it. Another meaning of trailer is a film preview or advertisement that theaters show before a movie. It might seem strange to call it a trailer when it comes first, but the earliest trailers, at the start of the 20th century, came after the movie, trailing it.