The forums are still full of topics about how to fill in the keywords in Meta keywords tag: comma or space, whole phrases or just words, etc. So as not to describe the article in a huge canvas, I'll say at once - the field or html tag Meta keywords should not be filled at all!
Previously this tag was useful and search engines took into account its content when ranking, but now this tag is not only useless, but you can get a penalty for overspamming keywords in it.
I have repeatedly conducted experiments with disabling Meta keywords tag altogether, and after that the position of the site did not fall, and sometimes even grew up. You don't have to take my word for it, but I think Google's head of anti-spam, Matt Katz, should be listened to:
The detailed Google documentation can be found here.