A primary keyword, as the name suggests, is a keyword that is used before any other keywords on a web page or in an article. Therefore, the primary keyword is the most important keyword on a web page. In turn, then, the primary keyword should be included in a site’s title and domain as well as in its content. It should be added to the first sentence on the page too. The primary keyword is essential for purposes of SEO as it is used to describe the web page as well as help people locate the site.
A secondary keyword is used after the primary keyword and is not required with respect to SEO. However, when a secondary keyword is used, the chances of attracting additional visitors to your site are increased.
A tertiary keyword falls third in line of importance and is used behind secondary keywords in keyword meta tags.
One should get to google for web tools to find and learn more on web tools that would enhance your knowledge of how best you can utilize web tools to SEO rankings.