A common practice among traders, traders borrow money from the brokerage to purchase stocks. The firm, in this case Robinhood and its banking partner, acts as the lender issuing the money based on account balances, creditworthiness, and other criteria. By artificially increasing the account balance the traders were able to get their hands on more money to purchase stocks.

https://www.forbes.com/sites/donnafuscaldo/2019/11/06/robinhood-glitch-lets-traders-borrow-unlimited-funds-to-buy-stocks/#6db74ce22125