The preview tab lets you configure how CrushFTP generates thumbnail images for the WebInterface. There is a presets drop down menu that should work for almost every OS. For Windows, you need to have 'ImageMagick' pre-installed, and possibly GhostScript as well. For OS X, just use the 'qlmanage' method. OS X could also use imagemagick if you prefer, but you would need to install that manually yourself.
So the top half of the window in general you don't touch or change anything. Just use the preset menu. The bottom half of the window, you specify directories that CrushFTP should monitor for new files. When it sees a new file, it generates a thumbnail for that file.
You can also have thumbnails generated for movie files. OS X has a built in method for doing this, other OS's can use other utilities such as FFmpeg. Still frames will be grabbed out of movies and made available as the mouse hovers over the icon.
The WebInterface with previews being generated may look something like this.