Bond prices and interest rates generally move in opposite directions. When market interest rates rise, existing bond prices generally fall, and when rates fall, existing bond prices generally rise. The reason is simple: if new loans pay 5% and yours pays 3%, nobody will buy yours from you at full price.
How much a bond moves when rates change is called duration. It is roughly about how long your money is tied up. A fund of three-month Treasury bills barely flinches when rates move. A fund of thirty-year bonds can fall by double digits in a year of rising rates. That is interest-rate risk.
Credit risk is completely different: the chance the borrower does not repay. The US government has always repaid in dollars, so Treasury funds carry very little credit risk. A high-yield corporate bond fund holds loans to companies rated below investment grade, where defaults genuinely happen — that is why it pays more.
These two risks do not travel together. A high-yield fund can hold short bonds (low interest-rate risk, high credit risk). A long Treasury fund is the mirror image. 'It's a bond fund' tells you almost nothing on its own; you have to ask which risk you are taking.
The yield curve is just every Treasury maturity's yield lined up from shortest to longest. It shows what the market is charging the government to borrow for three months versus thirty years, and it is watched closely because it shifts as expectations about rates and the economy change.