Brian the banker pays $100 for a 30-year “instrument” that pays $2/year plus $100 at the end. ($160 total) That night he changes his mind and wants to sell it. No problem. Interest rates are still 2% so he an still sell if for $100.

But,
what if the next night interest rate rise to 4%?
If that happens then every potential buyer can now take $100 to the credit market and buy an “instrument” that pays $4/year plus $100 back at the end. $220 total)

No Way is anyone gonna pay Brian $100 for something that pays a total $160,
not when, for the same money they can buy a thing that pays $220. Nope, rates up, bond values down. No one’s gonna pay Brian $100 for that “instrument” anymore. Brian just lost value.

This could be bad because, um well, the money Brian used to buy the bonds wasn’t his. It was his depositers’ money, and Brian’s depositers could ask for their $100 back at any time.