Somebody else will be able to explain it better, but in short, some big name short sellers are attempting to drive down the share price of Gamestop, which would make them a lot of money at the expense of shareholders. Some savvy redditors on WSB realized that there were a ton of people doing this, and somehow managed to unite pretty much all of WSB in buying into GME, which is driving the price way up in something called a short squeeze.

What this means is that those big money investors that are attempting to short sell the stock are losing a lot of money. Short selling is a bit of a scummy practice, and this ordeal has evolved into a battle of small time retail investors vs big money investors. It's been very entertaining.