Answer:
The major bond market in the United States in the market of U.S. Treasury notes. These bonds are issued by the US government and the market is very large because they are considered very safe, even if their yields are relatively lower than other bonds.
The yield of the bond market is determined by the price of the bonds. While the interest payments are mostly fixed, the yield can vary a lot because the price of the bonds varies a lot, depending on many factors, especially the interest rate, and the discounted cash flow expected from the bond.