Some very rich people shorted Gamestop stock. Shorting is where they borrow stock from someone on a contract, they have to pay a fee to borrow the stock and return the stock on a certain day. The person who borrows the stock sells it (say for $10 per share) and hopes the value of the stock falls (to say $5 per share). When they have to return the shares, they buy them back for a lower value and return ownership to the person who lent then the stock, and then get to keep the difference from what they sold the shares at to what they had to buy the shares back at. A bunch of people figured out that somebody borrowed a lot of shares of Gamestop and sold them (for say $10) and was hoping to buy the shares back for $5 and would get to keep the difference. What this bunch of people did was all start buying shares of Gamestop. Everyone on the market sees that there is a big demand for these shares so the value of the shares goes up. Now everybody is holding onto these shares, not selling them, driving up the value even more. On Friday, the people who borrowed at $10 and were hoping to buy back at $5, will have to buy back at $250 or more, which is going to cost them billions of dollars.