Keywords stuffing are when you write text that is filled with lots of keywords but basically no content in order to attempt to fool search engines into thinking that your website is about some subject. This is a really bad idea, though, because in writing a blob of keyword text, you have made your website less useful to humans, and how relevant your website is to humans is what any good search engine will rate you on. So, in short, make your website relevant and useful, and SEO will automatically follow.