See description tag.

The keywords tag is Meta data that appears if you right click your mouse and click "View Source" on any Web page. Although this tag is not supported in many engines (this is not true with some Trusted Feed campaigns), the keywords tag is a place to highlight your keyphrases and include misspellings. many search engines take this as a very important aspect in ranking a web page. so for optimization you should consider this very carefull. This data is written like a marketing statement and should clearly communicate the purpose of the page. Ideally, the description tag should be no more than 170 characters (with spaces). For instance, the keywords tag for this page appears like this:meta name="keywords" content="search engine optimization, Internet marketing, XML feed, conversion rates, landing page"

META tag used to help define the primary keywords of a Web page