Keyword stuffing is simply the process of repeating the same keyword or keyphrases over and over in a page. It’s counter productive. It’s is a signpost of a very low quality spam site and is something Google clearly recommends you avoid. It is:
“the practice of loading a webpage with keywords in an attempt to manipulate a site’s ranking in Google’s search results”.
It makes your text often unreadable. It often (really often) gets a page booted out of Google but it depends on the intent and the trust and authority of a site – and I’d think it would be diferent rules for different verticals (perhaps). Its is sloppy seo. It is a bastian of useless seo companies.
It might get a page to rank for a while, but will it convert any customers to leads and sales? It is obviously not a tactic you want to employ in search of long term rankings.
It can be a bit of an art though. There’s clearly a tipping point, very difficult to identify. Just because someone else is successfully doing it do not automatically think you will get a way with it – page penalties really do depend on the site you do it on but penalties are common.
Don’t do it – there’s better ways of ranking in Google without resorting to it.