Content information about all the videos can be found in the description. Trailers perform a different function. They show the style of the videos and how the author communicates with viewers. They also have a call to subscribe to the channel quite often.
A trailer is displayed on the main page:
It is recommendable to use cuts from existing videos on the channel, add a slide show with screenshots, or make a separate video. Big channels release trailers of new seasons and new episodes. It is played automatically for users that haven’t subscribed yet and gives the idea of the content of the channel.
To add a trailer, you should find the For New Viewers section on the Videos tab and click the Add a trailer option.