Monthly Cash Flow = Effective Rent (after vacancy) − Mortgage Payment − Property Tax − Insurance − HOA − Management Fee − CapEx/Maintenance Reserve − Utilities. It's the single number that most directly answers "does this property pay for itself, and by how much?"
A property can have a solid cap rate but weak (or negative) cash flow if it's financed with a small down payment relative to the purchase price, since more of the rent goes to debt service. Always model cash flow with realistic vacancy and maintenance reserves included — not just the mortgage and taxes — to avoid an overly optimistic picture.