A “put” is basically the right, at some point in the future, to sell a stock at the price that it is at when you buy the put. It doesn't actually involve buying or selling any stock at the time you buy the put.
You buy the put, hoping that the stock price will go down. If it does, then when the time comes, you can buy that stock at the now-lower price, and then sell it right away at the higher price that was in effect when you bought the put.
The opposite of a put is a call, which establishes the right, at some point in the future, to buy a stock at the price in effect when you bought the call. You hope, then, that the price goes up, so that you can buy stick at the price in effect when you bought the call, and sell it at the then-higher price.
An even more extremes bit of trading is to sell short. You borrow stock, and then sell it right away. You now have the money that you got for selling the stock, but you owe someone that stock. You're hoping that the stock will go down in price, so that later you can buy the shares of stock to repay what you owe, for less than what you got for selling the borrowed stick in the first place.
