Interest rates are a measure of the value of money and determine how much of the principal the borrower will pay to the lender over a set period of time. It is expressed as a percentage and, in simple terms, tells us how much extra the borrower will pay.
Usually it is a fixed (fixed) interest rate, but sometimes it is variable, i.e. it can change during the course of the loan. Banks have the possibility to set individual interest rates, not only for regular loans. You may also find individual interest rates for mortgages.