If I buy 100 shares of Gamestop and then I sell them to you and promise to buy them at a later date. Not just a promise - I legally have to buy them back.
In meantime, I'll pay you interest on the shares.
So on/by that date, I think the value will go down by so much that I'll be able to sell the share back to you at the lower prices and pay the interest. The difference is my profit.
So - If I sold my Gamestop shares to you for £10 each and the on that date I have to buy them and the price has now dropped to £4.
I've made £6 per share minus interest.
However, users in this Reddit thread buy Gamestop shares and drive the price up. The price it can go up doesn't have a cap. It could go up to £1000 per share if people keep trying to buy them.
So on that date, I have to buy them back at £1000 each when I only sold them to you for £10 and I still need to pay you that interest as well.
Betting on the shorting an old high street retailer like Gamestop seems a safe way to assume the market would move. Trading has been bad despite the games market being strong. It was very difficult to get hold of a PS5 they were selling so fast. You can only buy directly from Nintendo now they don't sell wholesale.
There's a great movie "The Big Short" where a small group of US investors borrowed shares in housing thinking the housing market would crash. Most banks were more than happy to lend them the shares as they assumed it would never happen and then they can make money on the interest as well.
A great film to watch if you're in the mood for some stock market movies after this.