Cash flow is the money left over each month after collecting rental income and paying all expenses including mortgage payments, property taxes, insurance, maintenance, property management fees, and vacancy reserves. Positive cash flow means the property generates more income than it costs to own and operate. Experienced investors analyze cash flow both on a monthly basis for operational planning and annually for investment performance evaluation. Consistent positive cash flow provides financial stability, funds reserves for unexpected expenses, and generates the income stream that makes real estate investing sustainable long-term.