The keyword density tool is useful for helping webmasters/SEO's achieve their optimum keyword density for a set of key terms/keywords.
Keyword density is important since search engines use this information to categorize a site's theme, and to determine which terms this site is relevant to -- the perfect keyword density will help archive higher search engine positions. Keyword density needs to be balanced correctly (too low and you will not get the optimum benefit, too high and your page might get flagged for 'keyword-spamming').
This tool will analyze your chosen URL and return a table of keyword density values for one, two, or three word key terms. In an attempt to mimic the function of search engines spiders, it will filter out common stop words (since these will likely be equally ignored by search engines), it will avoid filtering out stop words in the middle of a term (for example: 'designing with CSS' would get through, even though 'with' is a stop-word).