TLDR News US on How Reddit broke the stock market?
Big investors borrowed lots of GameStop stocks in anticipation that the price will drop (they "shorted" the stock). But the price did not drop. The big investors "shorted" further expecting that other market participants would follow suit in selling GameStop stocks. They didn't and then trolls from reddit joined the fray...
No one knows when and how this will end, but some people on reddit will earn a lot. While some big capital investors will hopefully loose a lot and learn a lesson.
Detailed explanation of what is happening in the following video by the TLDR News US.