Quantitative Easing (QE) is when a central bank buys large quantities of bonds (and sometimes other assets) to inject money into the financial system and push borrowing costs down — typically used when interest rates are already near zero and there's no more room to cut. Quantitative Tightening (QT) runs in reverse: the central bank lets those holdings shrink (or actively sells them), pulling money back out of the system.
For currencies, QE tends to be associated with currency weakness (more supply of the currency, lower yields), and QT with the opposite. But the relationship is rarely as clean in practice as with interest rate moves — QE/QT operates over months and years, and markets often react more to the *announcement* of a change in pace than to the ongoing mechanics themselves.