It refers to a brief list of most important keywords and phrases of your webpage. They are not visible to the users like your meta titles and meta descriptions.
Meta keywords are placed in the HTML code of the webpage. They tell search engines which keywords your page is optimized for.
To write meta keywords scan the copy on your page and make a list of important words or phrases then shortlist ten to fifteen keywords which you think accurately describe the content of your page.
Some instructions to write optimized meta keywords are listed below:
Meta Keywords : Syntax