Shorting a security is borrowing a security and selling it based on buying it back at at an anticipated discounted price in the future. The difference in the initial sale price and the discounted future price is the profit of such a deal.
Borrow and sell at $100/share, price dives to $75/share, short seller buys back at $75 for a $25/share profit.
If the security rallies, and the price goes up the short seller losses the amount above the borrowed selling price.
Borrow and sell at $100/share, price rises to $110/share the short seller then has to buy back the security at $10 more per share and loses money covering the short position.
Short sellers cover their shorts when the future price is closing in on the original borrow/sell price. At times short covering can lead to a stock rallying as shorts are covered. One can literally lose ones shorts shorting individual stocks.
There are Exchange Traded Funds or ETFs that one can buy into to short a segment of the market or one of the broader indexes.
With any investment there is risk, one has to asses their tolerance of that risk.
