Tapering, in the financial world, refers to the winding down of certain activities by a central bank

In October 2017, the Fed took the extra step of quantitative tightening. The Fed was no longer adding assets to its balance sheet every month, and it then decided to reduce the assets it held every month.

Beginning in September 2019, the Fed reversed course again and began adding assets to its balance sheet. 

Mixed feelings about quantitative tightening among investors could have added to stock market turbulence in 2018 (though many factors contributed to the turbulence).

https://www.thebalance.com/fed-tapering-impact-on-markets-416859

Dow

Taper Tantrum

https://www.investopedia.com/terms/t/taper-tantrum.asp