Simple interest is calculated only on the principal - the original amount of money borrowed or invested.
A typical interest rate is per year and the principal is usually borrowed or invested (and interest accrues) over a number of years.
p = principal
r = interest rate per period (expressed as a percentage %)
n = number of periods
Calculate the simple interest accrued on $100 invested at an annual simple interest rate of 5% over 2 years