Shorting Stock

 Shorting is borrowing stock from a broker on the expectation it will fall in value. 

Then after borrowing it, you sell it at the expected higher price. 

Then if and when the stock falls in value, you buy it back at the estimated lowest price. 

You pocket the difference between the high sell price and low purchase back price, and you give the initially borrowed stock back to the broker. 

The net profit will be that difference less interest on the borrowing and any fees and commissions.