"Keyword stuffing" refers to the practice of loading a webpage with keywords in an attempt to manipulate a site's ranking in Google's search results. Filling pages with keywords results in a negative user experience, and can harm your site's ranking. Focus on creating useful, information-rich content that uses keywords appropriately and in context.
To fix this problem, review your site for misused keywords. Typically, these will be lists or paragraphs of keywords, often randomly repeated. Check carefully, because keywords can often be in the form of hidden text, or they can be hidden in title tags or alt attributes.