For many websites, the way you structure the URLs and links on your site can have an impact on how the search engines will rank the site. Pages with URLs that are easy to read and include keywords tend to rank higher than those that consist of nonsensical numbers or letters. Specifically, many CMS systems produce URLs like www.example.com/?p=123. These are terrible both for search engines and for visitors trying to glean information about the page. URLs like that should be avoided.
For sites with more than a handful of pages, it often makes sense to categorize them into groups by using the same "path" to different pages. For example, if you have several products, you could group them under www.example.com/products/ to have www.example.com/products/blue-widgets and www.example.com/products/green-sprockets. Or, one level deeper, www.example.com/products/widgets/blue-widgets and www.example.com/products/sprockets/green-sprockets. Notice how the keywords "widgets" and "sprockets" are used (or re-used) without being obnoxious.
Not only does this help tell the search engine spiders that these pages are really about the sprocket and widget products, but it reassures visitors that this page really is what they're looking for.