Gamestop was predicted to go under so Wall Street decide to do something called "shorting" the stock. They borrow it from someone else and sell it and plan on the stock being sold back to them at an even lower price so they make a profit and hand the stock back to whoever they borrowed it from after making their money. Note they HAVE to give back the stock at the appointed time. Which is today, January 29th at time of posting. Thing is, R/Wallstreetbets bought out as much stock in companies like Gamestop, AMC, and Nokia, stocks that hedge funds bet heavily on making massive returns. Now they're held by Redditors who even if they refuse to sell, can make massive bank by selling a stock that cost say $20 for 6, 7, 8 times the price since the brokers HAVE to hand back the stock they sold but do not own back to the owners.
In short, Wall Street Brokers bet they could make money on industries being killed by the ongoing pandemic and government mandates. And with how things have played out, they are at MASSIVE losses in the billions of dollars, since if they want to make good on their promises and deals, they HAVE to buy the stocks at their increased price if anyone is willing to sell them. They've engaged in some shady manipulation (ie Market Manipulation, a big no-no) in an attempt to stop their losses or reduce them.
https://youtu.be/jg85H26wyLk