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Bonds are debt securities where investors lend money to governments or corporations in exchange for periodic interest payments and return of principal at maturity. Bond prices and interest rates move inversely; rising market rates cause existing bond prices to fall while providing entry points for new investors.
When you buy a bond, you are acting as a lender. You hand money to a government or corporation, and in exchange they promise to pay you a fixed interest rate (the coupon rate) on regular intervals and return your original amount (the principal or face value) on a specific future date (the maturity date). This is why bonds are called fixed-income instruments — the cash flows are predetermined and contractual, unlike stock dividends which can be cut or eliminated. The fixed-income concept from your prerequisites applies directly here: you already understand APR and APY, so you can evaluate a bond's coupon rate as an annualized return on the amount lent.
The most important and initially confusing property of bonds is the inverse price-rate relationship: when market interest rates rise, the prices of existing bonds fall, and vice versa. Here is the intuition: imagine you own a bond paying 3% annual interest. If market rates suddenly rise to 5%, new bonds offer better returns. Your 3% bond becomes less attractive — the only way a buyer will purchase it from you is at a discount, so the effective yield matches the new market rate. The bond pays the same fixed coupon, but because the price fell, that coupon now represents a higher yield relative to what was paid. The reverse is equally true: if rates fall, your 3% bond becomes premium — buyers will pay more for it, driving its price above face value.
Duration is the key measure of this price sensitivity. A bond maturing in 30 years is far more sensitive to rate changes than one maturing in 2 years, because the fixed cash flows extend further into the future and therefore suffer more when discounted at a higher rate. Short-duration bonds (short-term) are more stable in price; long-duration bonds (long-term) offer higher yields but swing more when rates move. For practical investing, this means matching bond duration to your time horizon: money you need in two years belongs in short-term bonds, while money you will not need for decades can tolerate the price volatility of longer maturities.
Different bond types carry different risk levels. U.S. Treasury bonds are considered nearly risk-free because the federal government can always raise revenue or money supply to repay; their yields are the baseline against which all other bonds are compared. Corporate bonds pay higher yields because corporations can default — the spread above Treasuries reflects the market's assessment of default risk. Municipal bonds (issued by state and local governments) often carry tax advantages that make their lower nominal yield equivalent to higher after-tax returns for investors in high tax brackets. Understanding which type fits your needs means integrating the yield, risk, tax treatment, and your investment time horizon simultaneously.