It's not a dumb question!
I heard a number of explanations about how bonds worked before it finally clicked.
First of all, it's really important to distinguish between bonds and bond funds (index or not).
If I buy a individual bond, it has a value it will be worth at maturity. It will also pay interest, either periodically over the life of the bond or when it matures.
**Assuming** the company does not default on its obligation and performs as contracted **and** I hold the bond to maturity, the bond does not lose value. It does what I contracted for it to do. It's worth what I paid for it.
Those are two really important assumptions. If the bond issuer defaults then the bond might drop to zero value or to pennies on the dollar at some future point after the company's assets are sold off to repay its debts.
If I suddenly need cash and need to sell the bond to get it, things get more complicated.
What interest rate is it paying and how does that compare to the interest rate being used on newly issued bonds.
Let's say I have a bond with a maturity value of $10,000 and it pays 2% interest. If new $10,000 bonds are paying 6%, who in their right mind would want to buy my bond at 2%?
The answer is, "Only a damned fool would pay $10,000 for 2% when they could buy 6% for the same price."
So, how can I sell my bond in a hurry then? The way to do it is to sell my bond at a discount. I'm not going to do public math, I'll just explain the concept. If I sell my $10,000 bond for less than $10,000, then the effective return on that bond goes up. If I give it the right discount, it will equal the 6% that newly issued bonds are getting. More of a discount makes it more attractive to potential buyers.
That's how individual bonds work.
So, how does bond fund change things?
First of all, it mitigates the risk that any given company goes bankrupt and I lose my entire investment. In that way, it's like a stock fund. Ditto, I'm buying a bit of many bonds instead of all of one bond, so it's much the same.
That's a definite improvement.
Where a bond fund is worse is what happens if I don't need to sell my bonds early for immediate cash but other people do (or just choose to do so). If enough other people want to sell, those bonds will have to be sold and I'll take a loss on my bond fund.
That's my understanding.
If other folks disagree, I would be happy to learn better!