In the stock market, for every buyer, there is a seller. When you buy 100 shares of stock, someone is selling 100 shares to you. Similarly, when you go to sell your shares of stock, someone has to buy them. If there are more buyers than sellers (demand), then the stock price will go up. Conversely, if there are more sellers than buyers (too much supply), the price will fall.
In short selling, the trader borrows stock (usually from his brokerage which holds its clients' shares or its own shares on account to lend to short sellers) then sells it on the market, betting that the price will fall. The trader eventually buys back the stock, making money if the price fell in the meantime and losing money if it rose. Exiting a short position by buying back the stock is called "covering". This strategy may also be used by unscrupulous traders in illiquid or thinly traded markets to artificially lower the price of a stock. Hence most markets either prevent short selling or place restrictions on when and how a short sale can occur. The practice of naked shorting is illegal in most (but not all) stock markets. How Do You Calculate 1 in Every?