The simplest, most direct explanation is that debt markets are an auction. Low bond prices are equivalent to high interest rates. If the Fed wants to lower in

The simplest, most direct explanation is that debt markets are an auction. Low bond prices are equivalent to high interest rates. If the Fed wants to lower interest rates, it buys bonds using money it creates, pushing up the price of the bonds, and effectively lowering interest rates. When new money is created, it dilutes the value of existing money, causing inflation. If the Fed wants to lower inflation, it needs to create less money. To do that it needs to buy fewer bonds. Less bond buying causes bond prices to go down, and therefore interest rates to go up. Now, the real world is a lot more complicated than this, and it depends on what exactly is happening with all of that extra money, but this is the first order explanation.