It's the "repo market," comprising the short-term funding that banks and financial counter-parties regularly tap to lend each other trillions.

It's suddenly in the news again, and for all the wrong reasons. The repo market is looking a lot like it did on the precipice of the 2007 housing market crash.

But what is the repo market, anyway, and why has the Federal Reserve Bank this week injected hundreds of billions of dollars into the financial system to stabilize it?

Repos (short for repurchase agreements) are short-term borrowing transactions, often made overnight. Think of them as trades of cash for some kind of collateral.