Keyword density is important part of a SEO compaign . It tells how often a search term appears in a text in ratio to the total number of words it contains. If a keyword appears two times in a 100 word text then the keyword density would be 2%.
Keyword density will be different for different search engines, however I recommend it to be less than 3%. If there is a keyword stuffing that a keyword appears too often in a search engines will downgrade the website and will appear lower in search engine.
You can check keyword density on various search engines like google, yahoo or zippy etc