Earning returns on both your original money and on the returns it has already produced.
With compound interest, the gains you earn are reinvested and go on to earn their own gains, so growth accelerates over time. It is the central force behind long-term investing: the earlier you start, the more time compounding has to work. The same force works against you on debt, where unpaid interest is added to the balance.
The Rule of 72 estimates doubling time: at 7%, money doubles in about 72 ÷ 7 ≈ 10 years.