The trailer is a feature that can be used to watch a video preview of your games in THP format.
Note: current WiiFlow 4.1.x do not play the trailers well. Hopefully will get fixed in the near future.
Trailers are placed in wiiflow\trailers from the root of your SD card or USB device. Only THP format is supported and they should be renamed using the first three letters of the game ID.
Once they are added, you can watch them by selecting a game and then press the "MINUS" button on your Wii remote.
If you want to create your own trailers and convert them to the correct format, you'll need an application called WiiFlow THP Video Converter created by mgrimmenator.
A complete tutorial on how to use this application and download link can be found here.
Trailers in THP format can also be viewed with SMPlayer under Windows and Linux.
Credit for these wiiflow trailers goes to spidy1000