The big hedge funds figured GameStop was going to go bankrupt, so they shorted the stock when it was like $8, figuring it'd go down in a year.
Some idiot on Reddit a year ago said "new consoles are coming out next year, and gaming is a huge industry and GameStop is the only physical brick-and-mortar player in the space" and I believe in that so much, I'm buying $53,000 worth of options a year out that it'll be at least $12 and maybe $18 by next year.
GameStop did pretty well in the year in between. They hired a new CEO, the guy who ran Chewy, which is one of only a few major companies out there that is beating Amazon in their retail space. The Chewy Guy is arguing GameStop can improve their online business too.
Then it was discovered that one hedge fund in particular was super-short on GameStop. There were 130%+ more shorts than longs... and all those shorts were going to have to buy GameStop stock by Jan 29th to make good on their contracts.
So reddit figured "if these guys have a contractual obligation to buy stock by Jan 29, and they need more stock than is actually available, and if we hold a bunch of stocks until they capitulate and pay whatever asking price we want for the stock, we can take this $20 stock and sell it to them at $200, $500, $1000 a share when Jan 29th comes and they have to buy from us."
When this word got out, it caused a bunch of people to start buying, making it even more expensive for the shorts to stay short. Now everyone is buying hoping that by Jan 29 they can force whatever price they want out of the shorts.
/WSB basically said "We're a bunch of idiot chimps flinging our poo around, but right now, we all just happen to be flinging it in the same direction"