(Reuters) - The inversion of the U.S. Treasury yield curve extended to 3-month bills for the first time since 2007. Here is what that means.WHAT ARE TREASURIES?U.S. Treasuries are bonds, or debt, sold by the federal government, most of which pay a fixed rate of interest over a fixed period, ranging from one month to 30 years.They are considered the world's safest securities because they are backed by the full faith and credit of the U.S. government.WHAT ARE TREASURY YIELDS?Treasury yields are a..