If you found this document with a search engine, you were probably searching for "getting found by search engines". That is because, for the most part, search engines search for the text that appears in the title of the document, and in the first part of the document. There are a great many more factors involved, some of which I will mention below, but for the most part, if the words 'getting found by search engines' had not appeared so many times, so early in the document, the search engine would not have rated it so high.
Note that if I had called the page "Pete's web page about getting search engines to rate your page more highly", It would not have rated so well because all the extra words 'water down' the importance of the document to the search engine. By the same token, if you were searching for just 'search engines', I would have been better to choose the title 'Search Engines' and not water it down with "getting found by".
There is a simple way of using this knowledge to get more people to find your site. I have, in the same directory as this page, a page called "Getting on Search Engines". This will attract the people who search for this sentence over "Getting found by search engines". But this other page, instead of having all the subsections on the same page as this one does, has a link to a subordinate page for each of the subheadings. Therefore, someone who would search for "meta tags" would not find this page, but the page to which the other one links. (generally search engines dissapprove greatly of duplicating pages with different titles. I mostly do this to demonstrate how splitting up pages into topics can help you to be found.)
Meta tags are a strange HTML construct which provide "information about information" (hence the name 'meta'). They are intended to provide search engines with information about the information contained on the page, without all the meaningless 'and's, 'but's, 'again's, 'sometimes's, 'e-mail's, and other ubiquitous generic words confusing the search engine as to the intent of the document.
More and more, however, search engines are placing less and less emphasis on the content of meta tags, as they can easily be abused: The classic example is of some marketing firm including 'child care' in the meta tag for a page that had nothing to do with child care, in the hope of getting lots of hits from people looking for something else.
Meta tags are good to have, for the sake of search engines that do pay attention to them, but remember that you must have the words you want to be found in your title and early in your document.
Updated November 6, 1997