When a bank makes a loan this is equivalent to the borrower issuing a bond. Many loans allow the borrower the option to repay its loan before term. This is equivalent to the borrower owning a call option. Many people complain when they decide to repay a loan early about the bank charging them a penalty fee. To them it seems grossly unfair. Penalty charges are completely understandable. The bank wishes to discourage early repayment and also has to be compensated for the value of the option it has written to the borrower.
The most obvious example of prepayment risks is that of fixed rate mortgages. These loans are long term and hence have long duration. When interest rates fall borrowers have a potential incentive to pay off their current loan and refinance it with a loan at the then prevailing lower interest rates. They are likely to do so when the savings from lower financing costs are greater than the costs of refinancing.