(More detailed version here.)
1. Some big hedge funds shorted more GameStop stock than actual shares outstanding. This is called a "naked short" and is illegal.
2. Some redditors noticed this, and bought the stock.
3. The stock soared as more and more investors bought.
4. To close out their short positions, those hedge funds must purchase shares of GameStop, even if it means suffering a huge loss. This is what's know as a "short squeeze."
5. Rather than sucking it up and taking their losses, many hedge funds not only doubled down on shorts, they went crying to the Biden Administration and their friends in the media that it was unfair they had been beaten at their own game.
6. Result: Ameritrade, Robinhood and others halt retail accounts from buying the stock, ostensibly due to "volatility," but actually to bail out the hedge funds.