What happened between Monday morning—when investors around the world were selling stocks as fast as possible—and today, when investors around the world were buying so furiously that they sent the Dow up by more than 600 points? Nothing, really. (Here, Wall Street people will recite various minor policy moves which are not the reason for these stock market swings, which happened due to arcane algorithims mixed with unbridled human fear and greed). People just changed their minds.
In conclusion, the stock market is driven mainly by bullshit. (In the short term. In the long term you should be fine until the inevitable unpredictable “black swan” event that brings this all to an end.)