Keyword stuffing is a bad practice in SEO or search engine optimization. SEO is a way to get your website to show up higher in search engine results, so more people can visit it.
When people type words into a search engine like Google, the search engine looks for websites that match the words they typed. Keyword stuffing is when people add a lot of the same words to a website in an effort to trick the search engine into thinking their website is more relevant.
So, if you were selling dogs and cats, you might write sentences like this on your site: "We sell dogs, buy dogs, purchase dogs, and adopt dogs!" That's keyword stuffing. The search engine would see all those words, but the sentences wouldn't make sense.
People might stuff keywords because they think it will make their website show up higher in search engine results. But it actually hurts the performance of their website, because when the search engine sees all that keyword stuffing, it takes away from the quality of their website. It's much better to just have one or two times a keyword appears naturally in a sentence.