The .ass file is the file containing the subs for the video in question, that's it it doesn't have the video in it which is why its so small. What you must have is both the .ass and the video file.
Ok once you have both of those downloaded, in a video player like VLC (download it if you don't have it) while playing the video you can right click and go to video then either subtitle or title (I can't remember what the menu is called in english, I use a Japanese version of VLC) then click browse. There you can search for the .ass file and it will load it into the video.
Really the only good thing about .ass files is that people who already have downloaded the video can quickly get the video going. The downside is they generally tend to be a lot less neater than pure hardsubbing. This is more due to the limitations of the filetype itself moreso than anything. However, some people can really make good .ass files.