Going short means borrowing shares and immediately selling it ,hoping to scoop at a lower price ,returning them to the lender and pocketing the difference. It is a strategy that is recommended for advanced/experienced traders.
To open a short position , a trader must have a margin account and will usually have to pay interest on the value of the borrowed shares while the position is still open.
To close a short position a trader usually buys the shares back at a price less than the borrowed and returns them to the broker /lender.
Short selling has a high risk/reward ratio, the profits can be huge but losses can mount up quickly and indefinitely.