Used mainly when searching, html tags tell search engines what the page contains making the searching process much quicker. When listing you site with some search engines, they ask you for a description of your site and some keywords that define your site, although it is the more powerfull search engines that actually extract the information from the meta tags on each of your pages to determine exactly what is on your page. Along with a few ather thing, it is the meta tags that must be 'tuned' correctly to make your site appear higher up on a search. Remember, META tags must appear within the header of the document, ie. For page transition META tags and transition codes, click here.
Some search engines index their lists on the site descriptions and sometimes the site titles.
For example, if your actual title was HTML Guide, search engines would pick literally thousands to millions of HTML Guide. A Simple HTML Reference Guide, the latter is what the search engine would list your site under. This is where the actual content, the actual description, keywords, title of your page goes. By adding this tag within the header tags, you are basically labeling the file as an HTML file to be viewed by anyone.