Compound interest has been called the eighth wonder of the world by Albert Einstein (source). Compound interest is the concept of earning "interest on interest". For example, if you have $100.00 that earns 10% annually, then the first year you earn $10.00 on your initial deposit. However, if you leave you're money in the account instead of withdrawing it, the second year your balance would be $110.00, so you would earn $11.00 in interest. And every year, you're interest would continue to grow.
Warren Buffet, also known as the "Oracle of Omaha", has earned a lot of his wealth due to the concept of compound interest. In fact, most of his wealth has been earned since he turned 50. You can see a great chart from MarketWatch in the article here. Warren Buffet has built a lot of weatlh over time by buying under valued businesses and reinvesting all the dividends and earnings back into the business.