Monetary policy is like a game where grownups decide how much money everyone can have and how much it will cost to borrow that money. The grownups in charge of this game are called the Federal Reserve, or the "Fed" for short.
The Fed makes rules about how much money banks can lend, how much interest they can charge, and how much money they have to keep in the bank to be safe. They do this by changing something called the "interest rate", which is the cost of borrowing money.
When the Fed sets the interest rate low, banks can borrow money more easily and for less money. That means people and businesses can borrow money too, so they might buy more things or invest in new ideas. This helps the economy grow.
When the Fed sets the interest rate high, loans become more expensive and fewer people want to borrow money. This can slow down the economy because people and businesses are less likely to spend money.
The Fed is also in charge of making sure there's enough money in circulation, or enough money for people to use to buy things (like toys, food, and houses!). They do this by buying or selling government bonds, which can affect the amount of money banks have to lend and the interest rates they offer.
Overall, the Fed's job is to try and keep the economy healthy by balancing the amount of money available to borrow and the cost of borrowing that money. It's like a big game of money-chess, but the moves they make can affect all of us!