"Selling short" means selling securities (stocks usually) before you own them.
How is it possible? Actually what you do is borrow stocks from others, sell them in the market, and buy them back when the price goes down making profit. Of course if the prices go up, you will end up incurring loss, which is the difference in prices plus interests and other administrative fees. So, it would be a smart thing to do if you could time travel back to the Crash!