basically
big rich old guys betted a lot of money for gamestop's stocks crashing because everybody buys games online
reddit heard about this and got a plan
if they randomly dump a bunch of money into gamestop's stocks it won't crash and all the money the old rich guys betted against it will go to them
also since they will be putting in a bunch of money into the stock the overall stock value will go up and make them more money again
so they teamed up and took all the old guy's money in the name of gamestop

Webtoon to understand better the GameStop thing?