Shorting stock is a form of speculation where you borrow stock and sell it at its current price with the hope that it drops and you scoop it up when it becomes cheaper and you pocket the difference.

eg. borrow 100 shares @$100 (sell for $10000), suppose stock drops to $50, you then buy back your borrowed shares (buy for $5000) and you get $5000 profit. However, if the price of the stock were to rapidly increase, you would be caught in a "short squeeze" and have to buy back your stock at the inflated price, but when you buy stock, it temporarily increases the stock price which worsens the squeeze and forcing you to buy back more at an even more inflated price and this quickly spirals out of control and the stock can reach ridiculous heights. For example, if it reached $1000 per share, then they would be forced to buy it back for $100000 which results in a loss of $-90000. The higher the price goes, the worse the loss is.

There's no theoretical upper limit to how high the stock can go and the higher it goes, the more expensive the buyback is. This makes shorting much more riskier than margin because you there's no theoretical upper ceiling to how much you can lose (with a margin call, the most you can lose is what you put into it)

So with regards to Gamestop, some hedgefund guys shorted it with the expectation that it would fall so they could make some profit. Reddit (r/wsb) for whatever reason jumped on this and started buying this stock en mass, causing the stock to rapidly increase and hoping to force a short squeeze. If the squeeze happens then the hedgefund guys lose a ton of money and redditors gain a lot and if the squeeze doesn't happen, eventually it will fall back to its pre-hysteria level and the redditors will lose a lot of money.

Right now there's a battle between the reddit nerds and the hedge fund people and it's unclear if a short squeeze will happen or if some billionaires will come in and shut this down.