Keyword Density is the number of times a Keyword or Keyword Phrase is used in proportion to the other words on the webpage. Keyword Density is a ratio and is almost always expressed in percentages, such as 2.5% or 10%. Basically, the more times a keyword appears the higher the density.
Although you want to include a keyword phrase, or search phrase, often you cannot use it too often. For example, if I included the phrase “Search Engine Optimization” 22 times in this blog entry Google would consider it spamming. If you are caught spamming or keyword stuffing your site could be penalized or dissappear from Google completely. These unethical practices will not only upset Google, it will also irritate potential clients/customers. If the content of your site is filled with junkie writing you will look unprofessional and untrustworthy.
Hiring an SEO Copywriter that understands how to properly integrate keywords into your site is always an easy option.