When it comes to displaying search results for a given search, the search engines display a few lines of relevant text for each item found.

In normal results (not for local map results), the first line is the title of the site (normally as defined in the html tag ‘Title”), followed by the website name. Immediately under the website name are typically two lines of text which are the ‘description’. Google describes this as a ‘snippet’ of relevant text describing the website.
 

This description is very important, as potential website visitors usually make an instant decision whether to go to a website based on how relevant the text is. Often sites which sound like another directory or poor quality affiliate site are passed over for a better sounding website.

Treat this description as a ‘mini-headline’ offering potential visitors both relevant content and a benefit to visit your site.

By adding a quality description into the html ‘description’ tag, you can help a visitor, rather than allow the search engine to pick some text to use in the description, which may not be what you want.