Short selling occurs when an investor is selling a stock that they do not actually own. When a stock is sold short, the individual doing the selling must deliver borrowed securities to the buyer by the settlement date. If a stock rises, the seller gets squeezed and will have to buy back shares at a higher price than they borrowed them to cover their position.
For this reason, short-sellers look to buy stocks that look to have minimal upside potential because that increases the likelihood that the stock price will decrease.