What does "Keyword Stuffing" mean?

Keyword Stuffing:

KeyWord Stuffing [KWS] is just what it sounds like; stuffing your keyword phrases into every possible place on a page. A little of this goes a really long way. It worked really well back in about 2005 as an obnoxious SEO technique. Now, Google penalizes or fails to reward (same practical difference) websites that have put their main keyword phrase into a page too many times. Avoid KeyWord Stuffing!

 Wiki: Keyword Stuffing

 Google Dev: Irrelevant Keywords