The META tags are used to provide extra information about a web page. There was once a time where a good search engine ranking could be achieved by simply changing value of META tags. However these days are long gone. Nowadays META Tags are decreasing in importance in eyes of search engines who are using more sophisticated methods of ranking pages (namely link popularity, optimized anchor text).
There are many different types of META tags but one that can affect ranking are Keywords META tag and Description META tag.
Keywords META tag:
This tag contains keywords relevant to a page. In past search engines referenced this to determine how to rank a particular page. However many people started to abuse this and stuffed tag with keywords that were not relevant to content of page. This confused search engines and subsequently returned weaker search results. To combat this less and less emphasis has been placed on Keywords META tag. However this does not mean that it is entirely useless. Here are some reasons to still include it:
* Some search engines still use it
Although most search engines do not use tag there are undoubtedly still some that do. For this reason place around 15-20 keywords relevant to your site or page into tag. Do not attempt to cheat search engines by repeating keywords as this is considered spamming and they may look unfavourably on this.
* Account for misspellings of keyphrases
People can misspell certain keyphrases that you may be optimizing for so it is common to place these misspellings into Keywords META tag. This allows you to rank in search engine results for misspellings without having to represent them on actual content of your page.