Meta Keywords Tag is used in the <head></head> section of a web page. In my personal view its of no use now a days in SEO. Although i had herd but not seen that yahoo still use this tag for its rankings.
Format for Meta Keywords Tag
<meta name=”keywords” content=”important keyword, comma separated, Other keywords” />
It is recommended to have 4-5 Meta Keywords related to the content of your web page if you are willing to have this tag in your web page.