The meta keywords element dates back to 1995. People were still searching the web using AltaVista and Infoseek and the web was still gloriously innocent. You could add a meta tag on a page, indicating that page’s main keywords.
This meta keywords element (like all meta elements) is invisible to visitors but visible to search engines. The keywords you put into this element’s content attribute would then be used as a ranking factor by the search engines.
If this sounds like you’re asking Google directly to just rank you for whatever keyword you want: that’s what it was. If you think “that’s too easy”: you’re right, it turned out to be. Of course, as the web became more commercial, people started spamming this keywords element and search engines began ignoring it.
It’s important to note that the meta keywords tag has nothing to do with the focus keyword in Yoast SEO. The focus keyword is a feature of Yoast SEO that is used to determine whether you’ve optimised your page well for the keyword you want to rank for. It’s not a signal that we give to search engines in and off itself.