https://www.vice.com/en_us/article/z3vgpx/institutional-investors-and-redditors-in-standoff-over-gamestop-stock

GameStop’s stock price is fluctuating wildly today. The stock hit more than $60 on Friday, a rise of almost 70 percent, and Wall Street halted trading of the stock. As trading began Monday, the stock surged to almost $150 a share. Since then it’s been a rollercoaster, with a horde of shitposters and WallStreetBets subreddit investors in a standoff against institutional short sellers and massive hedge funds to see who blinks first. 

As we reported last week, GameStop (which trades as GME on Wall Street) has been shorted massively, which means investors—most of them backed by big financial firms—have been betting on the stock going down in price. More stocks have been short sold than actually exist, which means that the short sellers have to "cover" the stocks at some point, meaning they have to buy actual shares of the stock. This is leading to something called a short squeeze, which is where an increased buying flurry both among normal investors and short sellers drives the price up.