If you haven't been paying attention to the persistent flattening of the U.S. yield curve, you're way behind it.

Peter Cecchini, chief market strategist at Cantor Fitzgerald, calls it "the most important thing to have a clear idea about now." Billionaire fund manager Bill Gross says we're rapidly approaching a point at which the trend will induce an economic slowdown. Others claim it's only natural, with the Federal Reserve raising short-term interest rates in the face of stubbornly low inflation.

To put it simply, the Treasury yield curve measures the spread between short- and long-term debt issued by the U.S. government. It's the extra compensation that investors demand to lock away their money for an extended period.