This is oddly fascinating, even though I don't understand all of it. If I understand correctly:
A "short" is a bet that a stock price will fall: you promise to sell it on a certain date at a certain price, but you don't actually own the shares. On that day, the idea goes, you'll buy the shares at the lower price you expect and then turn around and fulfill your contract, pocketing the difference. I don't know if regular folks like you and me can do that, or if only investment funds and professional stock-market people can. There are some rules that are different for the big players and the little folks; I don't know if this is one of them.
So... some big Wall Street hedge funds (one often mentioned is Melvin Capital) placed vast quantities of shorts on a gaming-gear company that isn't doing well (GameStop). A bunch of people on Reddit observed this and said to Wall Street: hold my beer.
They bought the stock. Hundreds of thousands of people on Reddit bought the stock. At that scale, any individual participant doesn't have to buy a lot; you could play this game for $20 back when it started. And it's not like you can spend that $20 going out to a movie right now, so there was probably an untapped market of bored people looking for fun.
Did I mention that this subreddit bills itself as "like 4Chan for investers"? And did I mention that Elon Musk tweeted about it to his 42 million followers? That subreddit has way more than "hundreds of thousands" of subscribers now.
What happens when lots of shares of a stock start getting bought? The price goes up. The price for GameStop shot up from less than $20 to, at one point, $347. And I think it was higher; I was only able to find daily closing prices, and the hour-by-hour swings have reportedly been wild. There's some background information on CNet.
The stock price, of course, won't stay high. It's a ridiculous price for that company, and eventually the market will bring it back down. But in the meantime, those hedge funds holding shorts have lost billions of dollars -- remember, they still have to buy the stock on "short day", at whatever price is then current, and then sell it for $10 or whatever the bet was.
The Redditors and crew, meanwhile, have turned their sights to other stocks; Blackberry and AMC have been mentioned as other companies in trouble that investors have considered prime candidates for shorts. Stock exchanges and Robinhood have stopped trading at times or restricted purchases.
By the way, the people rallying against Wall Street have a song -- a sea shanty.
I don't know what a "tendieman" is (Google has been unhelpful), though I assume it has to do with tendering, in this case selling at the right time. Ryan Cohen is a major investor in GameStop who's recently been investing more and trying to change the company's business strategy, though I can't tell if he has an actual position there. (The song implies he's on the board.)
As far as I know, the people organizing on Reddit and wherever else aren't doing anything illegal. They're not insider traders with privileged information -- quite the opposite. They're just...massively trolling big investors who traditionally make a lot of money with these kinds of bets. Some of them seem to be in it for the laughs; some are trying to make money riding this (but a lot of them will probably lose money, including anybody who tries to join in now). The line between a movement and a mob can be fuzzy; I'm not sure which this is. I wonder what the other damages are going to be. They're pitching this as little people versus big investors, but will little people with modest retirement funds end up taking some of that damage in those funds too? Or are hedge funds more esoteric and not usually part of IRAs and suchlike?
Bizarre, fascinating, and unsettling.
There's lots of interesting discussion in the comments on Dreamwidth.