Also known as a meta description tag. This is a short paragraph (HTML) that provides search engines with a description of a page’s content. This is used by the search engines for indexing. The description tag is not displayed on the website but is part of the header and is placed between the <head></head> tags.
The description tag is an advert to your site and is important to drive people to the site. For this reason, the description tag should be concise and compelling. The description is usually the first introduction that potential users will have to your site. In effect it is the 'call to action' before the page 'call to action'. In other words, the call to action from the description is to visit the page. The call to action on the page is to click somethng or to read-on.
While the description tag is an important part of the Google ranking algorithm (and therefore is important for SEO) it is important to remember that the description is constrained to 155 characters and will be truncated to the end of the last word before the 155 character limit by Google.