We like to think that when we deposit a dollar at the bank, it goes into a big vault and we can pull out that same dollar at any time. But that¿s not how the U.S. banking system works. Banks take that money and invest it to make money themselves, so cash gets spread around. This, naturally, leads to a big risk: What happens if those investments go sour? Well, you¿d be out of luck. You can¿t get your dollar back.

The Federal Reserve doesn¿t like that scenario, so it prohibits banks from putting all the cash it has on deposit on the line. In fact, the Fed forces banks to keep a portion of their assets at the Federal Reserve itself, to make sure that some of your assets won¿t get squandered if the bank¿s bets go south. These are called ¿reserves,¿ (hence, Federal Reserve. Got it? Good), and usually amount to 10% of the total cash kept in checking accounts.

These reserves are never exactly 10%, and banks like to keep a little extra in reserve ¿ not, as you might think, to make you more comfortable that they¿re in good financial shape, but rather so they can take that excess and lend it to other banks and make money off it. (They¿re banks, they can¿t help themselves.) The rate at which they make these loans is called the Federal Funds rate, which is set by the Federal Reserve¿s Federal Open Market Committee.

When you hear people chattering about how the Fed cut or hiked interest rates, this is what they¿re talking about: the interest rate banks can charge for lending money from their reserves. This begs the question: If these are essentially loans between banks, why is the Fed Funds rate so important for the rest of the economy?

Well, simply put, because loans make the financial world go round. Bank A lends Bank B $10,000 at a Fed Funds rate of 5%. Bank B then lends out $10,000 to a small business at 7%. The small business then takes that money and expands the business and hires new workers. Now someone is employed, Bank B has made interest off the loan, and Bank A is the richer for making it all happen. It¿s perhaps overly simplistic, but you get the idea. When you want the economy to thrive, you make lending cheaper.

Of course, sometimes you don¿t want the economy to thrive. In fact, you might want it to cool down, mostly to avoid money flooding the system and causing inflation. In that case, the Fed raises interest rates, making it difficult to lend or borrow.