A meta keyword tag summarizes all the keywords that you would like to be found under for search engines.
Meta tags are hidden in the document's source, invisible to the reader. Some search engines, however, do see and use them as a part of their algorithm. Engines do not penalize those who use meta tags properly, so it's recommended that you always include them on your pages
Avoid the use of spam
Avoid repeating keywords more than 3-7 times; you can get penalized for that. Repetitions up to 3 times are recommended, just don't place them one after the other.
Provides a list of keywords
Limit it to 1024 characters
List keywords in order of importance
Don't repeat the same keyword more than 7 times