Trailer is a word with two main meanings:

1. A vehicle designed to be pulled behind a car or truck. A camping trailer might have beds and a tiny kitchen inside. A boat trailer carries a boat to the lake. Semi-trucks pull huge trailers full of cargo across the country. Some families live in mobile homes, which are large trailers designed as permanent residences. The key idea: trailers don't have engines of their own. They trail behind the vehicle doing the pulling.
2. A short video preview that shows highlights from a movie or TV show to make people excited to watch it. Movie theaters show trailers before the main feature starts. When a new superhero movie is coming out, its trailer might show spectacular action scenes and funny moments without spoiling the whole plot. They're one of the most exciting parts of going to the movies, giving you a taste of coming attractions.