What is keyword density?

I am learning so many new terms in my search to become an internet guru. One term I keep coming across is "key word density". Key word what? Why is this important to me?

Wikipedia defines it as:
Keyword density is the percentage of words on a web page that match a specified set of keywords. In the context of search engine optimization keyword density can be used as a factor in determining whether a web page is relevant to a specified keyword or keyword phrase. Due to the ease of managing keyword density, search engines usually implement other measures of relevancy to prevent unscrupulous webmasters from creating search spam through practices such as keyword stuffing.

So there you go! Now prepare to be stuffed..