Keyword stuffing basically related to web page loaded with keywords in the meta tags or in content of a web page. The repetition of words in meta tags is considered to be unethical search engine optimization (SEO) technique, which leads to banning a website from major search engines either temporarily or permanently.
It is putting too many of the keywords into a page. Google does not like it and it has no value on your web page if you do it.
That's why there are some rules to put url in the pages,like if you are going through Web 2.0 there you can put limited keyword,same for meta tag,title tag,description etc....
When search engine crawl your site if he get Repeating keyword's in your keyword tag then it call spam or keyword stuffing... It's all effect your rank.
<meta name="keywords" content="Printed balloons, Custom Printing balloons, Custom balloons, Printing balloon">
In this keyword tag "Printing balloon" keyword are repeating, It's keyword stuffing..
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