You buy a bond from the government for $1000 paying 5% interest and, as long as you keep the bond, you're guaranteed to get that interest money and get your $10

You buy a bond from the government for $1000 paying 5% interest and, as long as you keep the bond, you're guaranteed to get that interest money and get your $1000 back when the bond matures. A bond fund has no "maturation date" therefore the only money you get back from it is the current market rate of the bonds in it which could easily go up or down.