The purpose of this tag is to define what keywords apply to your page. However, only some search engines will read this tag, and those that do often do not place as great importance on it as keywords found in other areas of the page. Still, you should generally include a META keyword tag on your page within the <HEAD> area.
Example of a META keyword tag:
<META name="keywords" content="blue widgets,green widgets,red widgets, Jerry's Widget Emporium">
Commas should separate each keywords or phrases. In general, list keywords in lower case. Avoid repeating keywords more than 3 to 7 times, and never list the same keyword twice in a row.