Bankers are beginning to discover that gamers are really freaking good at playing games once they figure out what the rules are. No matter what the game is.
What happened is a bunch of hedge funds shorted Gamestop. That means they bet on the stock dropping. If the stock doesn't drop to a certaint point in a certain timeframe then they need to make up the difference. Its basically betting on a company to fail. r/WallStreetBets noticed this and was feeling nostalgic about Gamestop and so started buying. That's what started its big climb and now more people are buying as the stock rises.
The stocks were shorted so badly, there were more shorts than actual stocks. So when the price rises due to buying pressure from the internet guys, those shorts have to be covered, meaning hedge funds must buy stocks at the hugely inflated price to cover their shorts. This makes the price go even higher. Major hedge funds are losing their asses. Melvin Capital are in so deep for the shorts that for every $11 Gamestop stock goes up they lose $1B, supposedly.
Basically, a bunch of Wall Street guys got greedy, bet against Gamestop, and a subreddit has cost them tens of billions of dollars and has them looking down the barrel at bankruptcy.