On Yoast's SEO plugin @ Wordpress, I can provide one search term per post, which is the key term, the focus term (don't know the English word for it). I am wondering what this keyword does...I don't see, if I fill it in, a thing related on it on my page source and I don't know what it does or what it should do.For example, I have a post called Hello World and I put in the key term: hello... What should Google do with this? Does it give priority? Does it do a thing or solely nothing? Can someone explain this to me? Because right now I have not filled in these fields, because I thought that it was unnecessary...  
WP Yoast SEO's focus search term; is it necessary?
seo;wordpress;keywords;search;plugin
The focus term (or phrase) is used by the Yoast plugin to check the page/post content for SEO against that specific search term.  So using your example of Hello World with hello as the term, the Yoast plugin will check to see if and how many times the word hello appears in theArticle Heading (any H tag in the body)Page or Post TitleURL slugContentMeta descriptionand based on those results plus an analysis of other factors (found under the Page Analysis table in the meta box on the post edit screen) the plugin will assign a red, yellow, or green light to the post where red is not well-optimized and green is well-optimized.  It does absolutely nothing to Google.  It is purely for your own quick testing to see if the page or post might do well against a hypothetical search.