Bonds and the Yield Curve
A bond is the simplest real instrument in finance — a contract to pay fixed cash flows on fixed dates — which makes it the perfect place to turn the time value of money into something concrete. But bonds also introduce the two ideas that run through all of fixed income: yield, the single number that summarizes a bond's return, and the yield curve, the market's price of time itself. Understanding bonds is understanding how markets discount the future.
A bond is the simplest real application of discounting: fixed future cash flows, priced by present value. From there come yield, the inverse price-yield relationship, duration and convexity as interest-rate risk, and the yield curve as the market's price of time. (Educational, not financial advice.)