Buying a bond makes you a lender rather than an owner. The coupon and the maturity date are fixed at issue, and the issuer owes those payments regardless of how the business or the economy performs.
The price moves inversely to yields. A bond paying a three per cent coupon is worth less once new issues pay five, and it must fall far enough for a buyer to earn the same five per cent to maturity. That relationship, not credit fear, drives most day-to-day bond price movement.
US Treasuries are the reference point for the entire financial system. They are treated as the risk-free rate against which every other asset is priced, which is why a move in the 10-year yield reprices equities, mortgages and currencies at the same time.