These tags are part of the HTML code. A Web author may surround sentences, even whole paragraphs, with these tags. Certain spiders then read the information in the tags as a way to help them index the site. You'd be surprised how many people are willing to do this. Once you are in an engine, check your site at least once a week.
Strange things happen.
Pages disappear from catalogs. Links go screwy. Watch for trouble, and resubmit if you spot it. Or simply join the Life Streams International family right now and receive free reports telling you where you are located in each search engine. The engines have grown smart enough to realize some sites only change content once or twice a year, so they don't visit regularly.