Keyword stuffing is the practice of listing keywords within the content of a page, or giving the keywords inflated frequency within the body of content, in order to appear relevant for those keywords and therefore rank higher in search results.
Informational retrieval science looks at the frequency of keyword appearance within a document, and compares it to all other relevant documents. Over egging the content with keywords can make a page look artificially relevant compared to other documents in the retrieved set. This can potentially lead to a ranking penalty for that document.