What is interest per annum?

[Jiub]

A couple hundred hours in and still no idea

[frank2351]

what you pay interest on your loans in a year

[OG Kiwi]

You take a loan and when paying it back, the amount required is a bit higher than what you were given. The added interest can be reduced by ideas, etc. to the point where it almost interest-free

[Kelldath]

The way loans are paid back in the game severely differs from what you commonly get nowadays from banks. You only pay interest during the loan duration, then have to repay in full at the end.
If you borrow 100 ducats, at 4% interest per annum. You'll pay 4 ducats per year for 5 years, then will repay 100 ducats in one payment after 5 years.
Total loan cost is thus easier to calculate: interest*duration*value.