What
Meta keywords are meta tags that you can use to give search engines more information about a page’s content. They’re found in a webpage’s HTML source code, and are not visible to visitors.
Why
Most search engines like Google do not use keywords from an SEO perspective. However some search engines do use them so I would add them.
How
Keywords are added to the meta tag keyword.
<meta name="keywords" content="PUT YOUR KEYWORDS HERE"/>
On this site they look like this:
<meta name="keywords" content="forest pathways, forest of dean, gloucestershire, web accessibility, website accessibility, jaws accessibility testing, supernova accessibility testing, adult accessibility testing, accessibility checker, accessibility law, accessibility help, web accessibility resources, web accessibility videos, manual accessibility testing" />
So whilst keywords are not used much longtailed keywords are. Read more here >>>>>>