Compound interest allows money to grow exponentially by earning interest on both the initial principal and accumulated interest. A $1,000 deposit at a 4% annual rate grows to $1,040 in one year, then ...

is—it’s effectively a percentage of an amount over a year. If your savings account earns you a 0.05% interest per year (which is a REALLY terrible interest rate, honestly), you earn $5 in interest for ...