Now imagine you had $10,000 to invest and could do so at 10% interest, but inflation was 20%. Your $11,000 only buys $8800 worth of goods at the previous years price.
Now imagine you are a bank that practices fractional reserves lending with $10,000. You can lend that $10,000 to several different people at the same time
You lend $50,000 of your $10,000 while still keeping $5,000 in the bank as a fractional reserve
Normally it would not make sense to charge less interest than the inflation rate, but banks when they do so can still make big profits. The borrowers get a good deal, they only pay 10% interest, but with inflation they pay the loans back in future cheaper dollars. You might even say they are being charged negative 10% interest.