I will explain it:
You have two types of orders in stock market exchange.
This much is clear, right?
We basically just buy a stock and sell it when price rises or falls. Now that is one way you execute your trades in stock market.
There are other ways to trade though. A short selling trade also known as "Naked Selling".
You basically execute a sell order first in the market where you don't even own the stock. You just sold it hoping to buy it later when the price falls hence getting you a profit. You must block a certain amount of money with the broker to execute such trades, either full amount of value of the trade or a margin amount. There are different rules for such trades that differ by countries and exchanges.
In India you must cover such a trade before the market closes. If you bought it in margin segment then you must cover the trade before 1 hour of closing of market or your broker will force a square off without your consent.
Demerits of shortselling:
If you sell a stock without holding it and by any chance you are not able to buy it back on that day then you are screwed big time. You will not be able to provide the delivery to the buyer and so next day your broker will buy the stock at a premium from the auction segment of the market and provide the delivery to the buyer. This way, you pay a lot of extra and it burns you.
If a stock you sold hits a circuit on the upper side, trading will halt and so it will become difficult to buy back the stock on that day. Next day, the broker will try to buy it from the auction segment paying a heavy premium which you will have to bear.
I explained it for Indian stock market, your exchange might have different rule. Also, this is for the cash segment and not the derivative contracts where you can sell a contract and carry your positions for several days.
Short selling could be beneficial for expert traders, I won't recommend it for someone new in the stock markets. I have been watching the stock market for 10 years and I won't do it. It's just not my style.