Black Hat alert! Keyword stuffing is a technique whereby you write content for your website that is overly contrived with an unnatural repetition of the same words.
The hope is that by having the same word repeated many times Google will think you are an authority on that keyword. But that's wrong.
Actually Google prefers to find content that is written naturally by humans, with the intent that it will be read by humans. Overly contrived pages are difficult to read. Google's built in grammar checker will detect and flag your page as keyword stuffed.
Although I've never heard of someone getting banned from search engines for keyword stuffing, the practice is automatically detectable by search engines and it works against you for attracting business to your site.
