Reinvesting Cash Flow - Question

[Matt Henderson]

I'm reading "Rental Property Investing" and Brandon mentions that you should "reinvest cash flow to have interest compound". Sorry in advance for a possible stupid question, however what is he suggesting to reinvest the cash flow in? He never says. Is this meaning to just save the cash flow and "re-invest" it in your next rental property? Just trying to understand what he means and where he means to reinvest the cash flow in order to receive the compounding interest which was described basically like investing in the stock market with an 8% return.