Keyword density is simple enough to understand, the more targeted keywords your page contains, the more likely that term will be associated with that page as a keyword. Since the keyword tag is more-or-less obsolete these days, keyword density is the way search engine spiders and crawlers determine what keywords that page will come up for.
Some things to think about when considering keyword density and your copy is that the more superfluous copy you use, the less dense your keywords become. Avoid using terms like "click here to view" on links or other such flowery verbiage to describe items unless a legitimate call to action is necessary. Look to your navigation links for an easy area to chop away at copy used across all pages of the site. The header and footer are other places to find copy to cut.
Keyword density in the title is also important but what's important to consider here is that the higher keyword density occurring sooner in the title, the more firmly associated those keywords will become with that page. In other words, start out with your top keywords early and often and leave the less important copy for the end. Also, be sure to keep the keywords within the first 70 characters to ensure maximum impact.