Keyword stuffing is an illegitimate, black hat search engine optimization technique that involves placing lists of search terms on the web page, in the title tag or within the meta keywords tag (although meta keywords are no longer used by Google’s algorithm to determine relevance) to “trick” the search engine into scanning and indexing the web page for all of the terms listed.
Keyword placement and density are important elements to search engine optimization. The idea is to keep your web page content highly focused on the relevant search term to avoid diluting its effectiveness by trying to optimize for too many terms. Even if the terms are related or mean the same thing, search engines don’t understand meaning they only see the word. For a more in depth explanation see my blog post Effective SEO Tip When Looking to Rank for Additional Key Word Phrases.