The Meta Description field on the SEO tab in the Big Tree CMS allows you to enter a short description of a page's content. While this description is not used by search engines in their ranking process to determine which pages should be displayed in search results, the page description is displayed in the search results themselves.
This text serves as advertising or marketing for your page or site, and so it can be beneficial to craft a compelling but readable description using imporant keywords. This can result in a higher number of searchers clicking on your entry in the search results (click-through rate).
Search engines generally only display the first 160 characters of the meta description, so it's best to keep your text concise.