Using a Meta keywords tag today
Because of the problem with allowing users to manually set pages keywords in Googles eyes keyword tags are no longer a standard within a web page and can be left out. Over the years Googles algorithm has gotten much smarter and complex and Google doesn’t need a keyword tag to figure out exactly what a pages and overall sites content in about. When I say left out it won’t mean any sort of penalty to you but however if you do place content within a keywords tag it will be read by Google and can appear in the SERP in certain situations.
The keywords tag as a reference
But as a tool to communicate the exact keywords your page is about, the keyword tag is no longer fit for this purpose. You can place your keywords here if you want to use the tag as a reference yourself but this is a pretty bad idea because people can see exactly what keywords you are targeting. Remember someone can easily check the source coding of your site to see how it is structured and the less information you make available about your SEO strategy the better. Don’t stuff a keywords tag with a pile of keywords for everything your page is about but if you do want to include it place one or two keywords in the tag that any SEO professional would know that page is targeting anyway. You can include keywords that are already in your Meta title if you want.