Placing excessive keywords into page copy and coding such as meta tags that may hurt the usability of a page but is meant to boost the page's search engine ranking. Hiding keywords on a page by making them the same color as the page background and loading tags with repeated keyword phrases are examples. Most engines penalize or ban sites that practice keyword stuffing.
The "Black Hat" practice of adding keywords and keyphrases to a page that have no particular value to a surfer in the hopes that the SE Bots will see them and that they will influence the page's SERP placement. Stuffed keywords may or may not even be visible to a surfer (i.e. black text on a black background, since a bot reads the code and doesn't "look at" the page, only the bot would see them)
The act of adding way too many keywords to a page in either the text or meta information. This is frowned upon by the search engines and regarded as SPAM.