A short has an unlimited downside. A put contract has a fixed downside, the price of the contract, which is paid regardless of whether you exercise the option or not.
A put allows you to sell an asset at a certain price in the future. So, say I bought a put option with a strike price of 10k USD for bitcoin 5 years from now. If in 5 years the price of BTC is less than (10k - the price I paid for the put option) I can buy a BTC for whatever it currently costs, sell it at 10k to whoever took the other side of the option and make a profit.
The risk is that if the price of BTC is above 10k, then I'm out the money the put option cost me.
Actually, if bitcoin becomes 15k then you don't have to pay anything. The put option gives you the "option" to sell a bitcoin for 10k (in this example), but you're not required to do so. In practice you only sell it if you'll make a profit. So in the case where bitcoin is 15k after 5 years your put option is worthless and you've just lost whatever the put option cost when you bought it.
True, yet the downside is still limited. The worst case for the option seller (or writer) is that she has to buy an asset valued at zero for the pre-agreed strike price. That puts a cap on her loss.
