Keyword stuffing is an old black hat seo technique which consist of inserting many times a keyword in a webpage or in meta tags for good SERP ranking on queries using the keyword.
When keyword stuffing is done (or rather was done) in page content. It has to be done without altering reading confort for visitors.
Different techniques were used to make the keyword repetition invisible for human and registered by search engine indexes:
bottom page placement
microscopic text size
same color for text and background
Coarse keyword stuffing belong to SEO prehistory as the practice has been easily, and for a long time, detected by Google and other search engines and may now induce some penalities.
Nevertheless, lines are blurred between keyword stuffing and density text optimization.