What is keyword stuffing?

Keyword stuffing is an outdated black hat SEO practice in which one chosen keyword is packed into the text of a website as often as possible, with the aim of reaching the highest possible ranking for that keyword in search engines. The name says it plainly: to stuff means to cram or to fill.

Why did keyword stuffing improve rankings in the past?

The algorithms behind search engines such as Google used to be far less clever than they are today. One of the criteria for good rankings was how often certain terms showed up in the text of a page, measured as a percentage: the keyword density. To push that percentage up artificially, a search term was worked into the text as often as it would go.

Search engines such as Google have come a long way since the 2000s and now spot keyword stuffing quickly.

What are the consequences of keyword stuffing for SEO?

Search engines such as Google have been spotting keyword stuffing since the 2000s and penalize websites in the rankings for it, because pages like these rarely offer any value to the people reading them. Next to a penalty or a complete removal from the index, keyword stuffing usually leads to a poor user experience: packing the keyword in again and again makes the text hard to read, and there is little room left for anything worth reading.

More on this at Google Search Central: Spam policies (keyword stuffing)
https://developers.google.com/search/docs/essentials/spam-policies

Not sure whether older texts on your site are still stuffed? An SEO audit goes through them page by page.