This is called a short squeeze.
The guys over at Reddit noticed “The percentage of the GameStop float sold short is at 139.57%, the highest level for any equity worldwide” Selling short means borrowing stocks, selling them off immediately with the promise to buy back (if as hoped the price drops) and pocketing the difference.
The guys a Reddit en mass pushed the stock price up, meaning if it was 10 dollars and went up to 100 to buy back the short would cost 100 dollars and the person who made the short loses 90 dollars and pushes the price up more since they are buying.
This is what happened to a few big firms whose liability was growing folded. Doesn’t mean its a good time to invest in Gamestop, unless there are a lot of other finance houses that will be forced to fold and buy back their shorts, because eventually the stock price will drop back down like a rock.