So OP, when the Fed wants to stimulate, it "buys" financial instruments (lately bonds, treasuries, mortgage-backed securities) by crediting the accounts of the big, system-enabling banks. This expands the money supply because the fed doesn't hand over existing money that it got from somebody else, it just adds the amount to the account of whatever big bank. (This is what people are talking about when they mention the money printer.) conceptually, it's like if you needed to borrow $100 and I mint a new $100 bill in front of you and then lend it to you. This is quantitative easing since it eases financial conditions: there's more money in the system, and that basic law of economics (greater supply reduces price) is true with money too (the price you pay is interest or the exchange rate, so larger money supply = lower interest ). It's also inflationary for goods and services since more money in the system means more money to spend on crap (increased demand = increased price).
Now, if the bond or treasury comes due and the fed doesn't simultaneously buy another offsetting bond/treasury of the same size, the opposite happens: the money supply shrinks because the fed debits the account of the big bank, which takes that money out of the system. Imagine my $100 loan from above is now due, so you pay me and I immediately light the bill on fire in front of you. That cash is now gone. Shrinking the money supply is restrictive (opposite of simulative) since it means there's less money to loan and spend on crap (deflationary), which raises the cost of borrowing, which pushes the economy toward economic contraction / a recession. That's quantitative tightening, or QT.
You tighten to fight inflation. Since the Fed's dual mandate says it must control inflation AND minimize unemployment, and they're ceasing their anti inflation action, this implies that 1) they're now increasingly worried about a recession and by extension unemployment (relative to their worry about inflation) or 2) they at minimum no longer as worried about inflation