The Meta Keywords tag is used to reinforce the keywords appearing on the web page. It is a list of keywords that search engines get for the page telling them what you think are the most important keywords on the page. Although Google is not using the META Keywords tag anymore, some of the other search engines might still give some weight to this tag.
Official Google Webmaster Central Blog: Google does not use the keywords meta tag in web ranking
What to write in the Meta Keywords field?
- Make a list of the most important terms you see in the body of the page, the META Title and META Description.
- Pick 5-10 of those terms as keywords
- Write each keyword / term separated by comma starting from the most important to the least important (Ex: "stamps, stamp collecting, stamp history, prices, stamps for sale");
- Or you can leave this field blank!