Hedge funds shorted the stock which means they borrowed stock from a brokerage then sold it at the market value. The hedge fund pays the brokerage interest on what amounts to a loan of stock, but eventually is required to return the stock. This means that they lose money if the stock goes up and make money if the stock goes below what they sold at. People are keeping GME expensive as fuck because they know that the hedge funds will need to buy it to return it to the brokerage at some point regardless of the market price of said stock. They could be looking at billions in losses because of this and if they are unable to make good it could cause a serious market failure.