Keyword stuffing is when someone lists as many SEO service keywords, irrelevant or not, onto a page in order to manipulate their page to rank high on the SERPs. This method did work at a point in time, but when Google figured it out, they updated their algorithm in order to stop keyword stuffing from happening and now penalizes websites who practice this method. Your page could either be demoted or even removed!
WordStream gives a great example of keyword stuffing in their article:
“Are you looking for cheap running shoes? If you’re looking for cheap running shoes, look no further. Our cheap running shoes website is the best place to order your new cheap running shoes. Feel free to check out our selection of cheap running shoes from our cheap running shoes selection below.”
I mean, if that example doesn’t make you cringe, then we don’t know what will!