Wikipedia's definition of Meta Elements:

Meta elements are HTML or XHTML elements used to provide structured metadata about a Web page. Such elements must be placed as tags in the head section of an HTML or XHTML document. Meta elements can be used to specify page description, keywords and any other metadata not provided through the other head elements and attributes.

In plain language, Meta tags are inserted into the page code to help search engines identify your page. Not all search engones use them but every site should have at least main site tags that will show up on every page's code. You can also create custom Meta tags for individual pages.

According to Wikipedia...

The description attribute is supported by most major search engines, like Yahoo and Bing, while Google will fall back on this tag when information about the page itself is requested (e.g. using the related: query). The description attribute provides a concise explanation of a Web page's content. This allows the Web page authors to give a more meaningful description for listings than might be displayed if the search engine was unable to automatically create its own description based on the page content. The description is often, but not always, displayed on search engine results pages, so it can affect click-through rates. Industry commentators have suggested that major search engines also consider keywords located in the description attribute when ranking pages.[6] W3C doesn't specify the size of this description meta tag, but almost all search engines recommend it to be shorter than 200 characters of plain text.[citation needed]

The keywords attribute was popularized by search engines such as Infoseek and AltaVista in 1995, and its popularity quickly grew until it became one of the most commonly used meta elements.[1] By late 1997, however, search engine providers realized that information stored in meta elements, especially the keywords attribute, was often unreliable and misleading, and at worst, used to draw users into spam sites. (Unscrupulous webmasters could easily place false keywords into their meta elements in order to draw people to their site.)

Search engines began dropping support for metadata provided by the meta element in 1998, and by the early 2000s, most search engines had veered completely away from reliance on meta elements. In July 2002, AltaVista, one of the last major search engines to still offer support, finally stopped considering them.

No consensus exists whether or not the keywords attribute has any effect on ranking at any of the major search engines today. It is speculated that it does, if the keywords used in the meta can also be found in the page copy itself. With respect to Google, thirty-seven leaders in search engine optimization concluded in April 2007 that the relevance of having your keywords in the meta-attribute keywords is little to none and in September 2009 Matt Cutts of Google announced that they are no longer taking keywords into account whatsoever.However, both these articles suggest that Yahoo! still makes use of the keywords meta tag in some of its rankings. Yahoo! itself claims support for the keywords meta tag in conjunction with other factors for improving search rankings.

On the Admin menu, go to Site > Global Configuration and on the first tab look for Metadata Settings: