A short tail key phrase is usually one to three words. However, it is difficult to get your post to the top of search results, so it works better to use a long tail key phrase of three to six words. The strategy recommend by Yoast is to use a short tail key phrase for a "cornerstone" article, an article that more broadly explains your topic. Then write several shorter in depth articles that explain your topic in more depth using long tail keywords. The long tail key phrases will help to strengthen your website's credibility for the short tail key phrases.