Not sure if that question is rhetorical, but on the off-chance it isn't, it's a kind-of .... stock market bet.
If most people, probably you and certainly me, want to buy shares in a company, you contact a broker, say "Here's £100k, (or whatever), buy me shares in (for example) Tesla".
Once the broker has your/my money, they buy however many shares that gets. When you sell, they sell at prevailing price, and then (after fees) remit you the proceeds.
But regular investors with whom the broker has a history and hence a degree of trust in the credit of, will do all that on account.
In other words, you issue the instruction to buy, and they buy immediately, and you then settle up at the end of the account period, which is when you need to produce that £100k.
So far, so good?
Right, "shorting" is selling on account, a stock you don't own. If you like, you sell "borrowed" stock, but then later on have to return what you borrowed. Such as, sell £100k of Tesla. But at the end of the account period, you'll need to own the stock you sold, because you have to transfer them to someone else to replace "borrowed" stock, so, within that period, you have to buy that number of Tesla shares.
So in effect, you sell sonething you don't own, and then have to buy it when time comes to deliver. Thus, you are effectively placing a bet that Tesla shares will have gone down in the meantime.
So, sell 1000 shares at £10 today, and you are in for £100k. In two days time, those shares are now £9 each, and you need 1000 because that's what you sold but didn't have, so it costs you £90k to buy them. But you got £100k for the 1000 you sold, meaning you have £10k (less fees) profit.
So, it's basically a bet that a company's stock will fall in price before you have to settle the account and transfer shares. Of course, get it wrong and you still have to provide the 1000 shares you sold, so if they're £15 ech now, it'll cost you £150k, meaning aa £50k loss on the bet.
Most companies don't like large scale shorting because volumes of "sell" orders don't do the stock price any good, and it distorts the usual market signals.