The story so far:
They are like the blurbs on book back covers that lure the author into wanting to read more. They should be short but intriguing. They are often the first look a reader has of your chapter and help them decide if they will read it or not. They shouldn't contain spoilers or a synopsis of the plot.
Previews are used in various places on StoryMash to provide readers with a, well preview, of your chapter. They also show up in the results of search engines such as Google