Example:
<meta name="Keywords" content="search engine marketing, web site optimization, search results ranking" />
This keyword tag is only important for your homepage as many search engines ignore this tag and it will not influence your ranking when missing.

However when keyword tags are present and keywords are used in a repeating manner it can have a reverse effect. It will be defined as trying to spam the search engines and as an attempt to fool them. This mostly results being penalized by search engines.