Keywords are the connection between your Website content and the prospective customer using a search engine to find you. The image below shows the Cache of a Website page stored by Google. Note how keywords are highlighted throughout the page. This image shows exactly how Google views a Website page in terms of its keywords.
Because of this connection between search engines and your Website, it is useful to anticipate what words or phrases a person would type into a search engine when looking for your Website or product. Think in terms of key phrases, not just key words because proximity between words is important. Tools such as Google Keyword Planner, Ubersuggest, and SEMrush help Webmaster's select relevant Keywords that arn't currently being used in high densities. Monitor how site traffic improves or declines based on which keywords are used, and adjust accordingly. Google Webmasters tracks search statistics and trends, and is an invaluable tool for keyword adjustments.
However, it is also critical to keep in mind that content is of higher priority to both Website visitors and Google Search Rankings. Consequently, analysis and any subsequent modification of Website content to optimize keyword density must still result in an article, blog or other Web page that is suitable for human readers.
Cautionary Notes from Google about Keywords and Content
According to the Google Webmaster Central Blog post “Google does not use the keywords Meta Tag in web ranking” the Keywords Meta Tag does not influence the search results ranking in Google. According to the post, Google stopped considering the keywords Meta Tag “Because the keywords meta tag was so often abused, many years ago Google began disregarding the keywords meta tag.”