Compound interest is the process where your investment earns interest not only on the original amount you invested, but also on the interest that has already been added. Over time, this creates exponential growth.
For example, if you invest $1,000 at 7% interest per year, after one year you would have $1,070. The next year, you earn interest on $1,070 instead of the original $1,000.
Over long periods of time, compound interest can significantly increase the value of investments, which is why it is often called "the power of compounding".