Keyword stuffing, also called over-optimization, is the excessive repetition of keywords in elements that contribute to SEO (titles, link anchors, ...).
Stuffing is a black hat practice that can be sanctioned by search engines. This practice can be harmful on several points :
The best way to fight stuffing today is to produce quality content combining the presence of targeted keywords for SEO and readability for the Internet user.
Do not hesitate to favor a developed lexical field rather than focusing on a repetition of your keywords, which can be considered abusive.
Indeed, Google and its algorithm now take into account the semantic richness of your content. This is why, even if you have not explicitly evoked certain keywords, you can still be positioned on a main expression and on secondary expressions, if your lexical field refers to them.