Keyword stuffing is the repetition and overuse of keywords on your page to try to gain an advantage in search engine rankings.
The following title tag is an example of keyword stuffing since it sounds very unnatural, and the word "blue widgets" is used repeatedly for no other purpose than to match search engine queries:
Blue Widgets, Blue Widget, Where to Buy Blue Widgets, Widgets that are Blue
While this technique may have been effective at one time, it is now easily detectable by search engines and considered to be a form of webspam.
Overuse of keywords to the point of keyword stuffing may lead to penalties and can hurt your rankings. Instead, natural usage of keywords and variations of keywords in moderation is encouraged.