Probably the best known and most commonly employed use of the meta tag is to provide information about one's site for spiders and search engines. The tags can be used to identify things like a description of the page, keywords to be used by search engines, and copyright information. The data you insert into these meta tags will be used by the search engine to create clearer, more concise results when someone is searching for a topic your site might fall under. However, there are no guarantees that the little spider will pull you into its Web of search results. Furthermore, depending on what the spider grabs from your site, your description on the search engines can become misleading.
Meta tags can give a little extra boost of visibility when the spiders come by.
In addition, depending on the content that you place inside your tags, your ranking in search results can be greatly improved.