Scroll down the page past the text box for your WordPress page or post, and go the the widget names Wordpress SEO by Yoast.
To begin, type in your Focus Keyword and a drop down list will appear on your screen. This drop down list happen when the Yoast widget is checking your page or post to see if your website's page or post is SEO friendly. Your goal as a web developer and designer is to get this drop down menu to say "yes" every criteria on their list.
Finally, to build a really strong page or post for your website, click on the tab called Page Analysis in the Wordpress SEO by Yoast box. This tab will tell you in detail how to create an SEO friendly web page or post beyond using a great focus keyword. Once again, your goal is the get the entire checklist to contain green buttons on all of the bullet points. Some great ways to enhance your page or post is to have a well written text on your page or post that is at least one paragraph long with well written sub-headers included , images with captions that are relevant to the subject, filling out the meta description in great detail with the focus keyword located inside, and have the URL contain the focus keyword.
If you are interested in learning more information about SEO and how to make your site more SEO friendly, please visit this WordPress SEO Tutorial