Similar to short calls, short puts (puts sold to open) are betting that the price of the underlying does not fall to the strike price of your sold put. Short puts also have a theoretically unlimited downside, with the maximum possible loss being reached if the underlying stock reaches $0 per share. 

Much like with short calls, short puts would give you an obligation to purchase shares at the strike price, in this case, if the value of shares is less than the strike price you sold the option for, you would be on the hook.