For those that don't understand how monetary policy works; here is an overly simplistic explanation.
The Fed has been buying billions of dollars in corporate bonds, treasuries, and mortgage backed securities every month since the start of the pandemic creating massive artificial demand for those assets and suppressed interest rates which has pumped trillions of dollars into the markets and economy that otherwise would not have been there. Most agree that it was necessary initially, but there is debate over how much and how long was appropriate. This is called quantitative easing.
The Fed is now tapering/reducing those purchases which means they are buying less assets every month with the intent to stop purchases all together by March or the end of Q1.
Now they plan to sell a good portion of the bonds that they have purchased that are sitting on their balance sheet. When the Fed sells assets they get cash in return which removes it from the markets and economy. This is called quantitative tightening.
No more fed pumping money every month, higher interest rates, and the removal of money from the economy is a huge change and cause for depressed market/economic outlook.
So basically, everyone is hoping the Fed can thread the needle and stave off significant inflation without causing a massive recession and stock market crash all while minding unemployment numbers and continuing to finance massive US budget deficits.
Sounds like a house of cards to me