The Google search engine moves the goalposts very often. Their reason is that they have to in order to react to black-hat companies that try to manipulate the system, and because they have to perpetually improve their search engine to give the best value. Others point out that every time they undergo a massive search engine change/update, the Google earnings suddenly shoot up. The truth is somewhere in the middle.
You may have read SEO (search engine optimization) advice that you should optimise for certain keywords so that your website ranks higher on the search engines for those words. The aim is to choose the keywords that your target audience are going to choose.