The meta keywords tag was an HTML element meant to let a page's author explicitly list the terms it should rank for, sitting invisibly in the page's source code rather than displaying anywhere a visitor would see it. In the earliest years of search, it carried real ranking weight.
That weight didn't survive contact with how easily it could be gamed: webmasters quickly learned to stuff the tag with dozens of high-volume terms regardless of whether the page actually covered them, which made the tag useless as a genuine relevance signal almost as soon as it became popular. Google confirmed years ago that meta keywords play no role in its ranking algorithm at all, and most other major search engines followed the same conclusion. The tag still technically works, browsers and crawlers won't error on it, but there's no SEO reason to spend time filling it in today; any effort that would go toward crafting a meta keywords list is better spent on content that actually demonstrates topical relevance instead.
