As the name implies, keyword stuffing is the packing of keywords. Since Google’s previous algorithm was simpler than it is now, SEO rankings were decided by considering how many keywords are included in the page. For this reason, there were cases where measures such as Google search were used to rank the text, meta-keywords, title tags, etc. on the page with keywords as often as an ordinary person would read, so that they could be ranked higher.
Although keyword stuffing used to have a certain effect in the past, nowadays the frequency of keyword usage has little effect on the ranking, so it is almost impossible to display it higher just by packing keywords.
Keyword stuffing violates Google’s guidelines and may be penalized for such measures.