Compound interest
Interest earned on both your original money and the interest it already earned, so growth speeds up over time.
With compound interest, each period's interest is added to the balance, and the next period's interest is figured on that bigger number. The longer the money sits, the bigger the effect.
It works against you on debt too. Unpaid credit card interest gets added to the balance and starts building interest of its own. Investment returns are not fixed like a savings rate, and they can be negative in some years.
Example
$1,000 at 5% a year: after year 1, $1,000 × 1.05 = $1,050. After year 2, $1,050 × 1.05 = $1,102.50. After year 3, $1,102.50 × 1.05 = $1,157.63.