Description meta tag is a meta tag used to describe the content of a webpage to search engines.
Description meta tag no longer affects ranking on SERPs, but it can sometimes be used by Google and other search engines to describe a page in SERPS (see video below).
A good use of description meta tag may therefore impact click rate in SERPs for a given rank.
Google guidelines relating to description meta tag:
Matt Cutts speaks about the use of meta tags: