Shorting stock is when someone sells a stock they don't own yet. It's like borrowing something from someone and then promising to give it back later.
The stock market works like a big store. People buy and sell stocks like things in a store. If someone believes that a stock's price will go down, they can take advantage of this by 'shorting' the stock. This means that they borrow the stock from someone else and then sell it. If the price does go down, they buy back the stock at the lower price and give it back to the person who lent it to them. They get to keep the difference in the price, which is their 'profit' from shorting the stock.