If you have some experience building websites, you're probably familiar with adding metadata information to pages. Metadata are added to the HTML document source code. It's information that's not displayed on the web page, but search engine spiders do process it. Search engines may present the content of the meta description tag in the search results page. Although meta keywords aren't of vital importance for major search engines anymore, it certainly won't hurt to add meaningful metadata to your site. In Joomla! 1.6, you have a little more control over the metadata that will be used in your site pages.
In Joomla! 1.5, the global site metadata were filled by default with some standard text about Joomla! itself. This didn't work out well: many site administrators didn't bother to change the default text, which meant that their sites were indexed by search engines using the wrong information. Instead of describing what the site was about, the metadata contained useless...