'Shorting' is, unbelievably, a legal operation.
The 'shorter' identifies a stock or currency that they think is going to fall in value. They then borrow as much as they can of it and sell it on the open market. When it has dropped in price they buy it back at the lower price, return, or pay back the stock or currency they borrowed and keep the difference in price between what they sold and bought at.
The man, Odey, who made a considerable profit by shorting the £ on Monday did the same thing when the value of the £ dropped on the result of the 2016 referendum.
Shorting shares as a result of insider knowledge is fraudulent. Shorting currency isn't.