Keyword stuffing is a spam technique that was popular many years ago. If you included a keyword often enough on a page, it was possible to get high rankings for that keyword.
Of course, keyword stuffing is against the guidelines of Google and Bing:
“When creating content, make sure to create your content for real users and readers, not to entice search engines to rank your content better. Stuffing your content with specific keywords with the sole intent of artificially inflating the probability of ranking for specific search terms is in violation of our guidelines and can lead to demotion or even the delisting of your website from our search results.”
Other search engines also don’t like keyword stuffing. It is something that should be avoided.
Google can handle keyword stuffing
Google’s John Mueller said on Twitter that, although Google does not like keyword stuffing, they are able to handle these pages without penalizing the website:
Yeah, but if we can ignore boring keyword stuffing (this was popular in the 90's; search engines have a lot of practice here), there's sometimes still enough value to be found elsewhere. I don't know the page, but IMO keyword stuffing shouldn't result in removal from the index.— John ☆.o(≧▽≦)o.☆ (@JohnMu) 20. Juni 2018
Avoid spam techniques
Although Google does not penalize keyword stuffing pages, it is better to avoid spam techniques to promote your website. The tools in SEOprofiler help you to optimize your web pages with white-hat SEO methods: