Site and Page Design
Keyword density means the ratio of keywords that you are trying to target for SEO purposes to the other text on your pages. Getting keyword density rightenough so that your SEO goals are achieved, not so much that the search engines are "offended"is a key goal of core SEO practice. Search engines do look for keywords, but they take away points for excessive and inappropriate keyword "stuffing."
Even from the point of view of your site visitors, you want a nice density of keywords in your pagesbut you don't want so many keywords that the content of your pages is diminished from the viewpoint of visitors.