A cash flow forecast is a projection of what will arrive and what must be paid, laid out by week.
Its purpose is timing. A business can be profitable and still be unable to pay a bill in a particular week, because the money owed to it arrives after the money it owes falls due. See profit and cash.
What goes into it
Money in: invoices due for payment, allowing for how promptly each customer actually pays rather than the terms on the invoice — see days to get paid.
Money out: payroll on its known dates, rent, loan payments, tax, insurance renewals, and supplier bills by their due dates — see bills you owe.
Why weekly
Monthly totals hide the problem. A month can end well and still contain a week in which the balance goes below zero, because payroll fell before a large receipt.
Weekly resolution is what makes the low point visible.
The horizon
Far enough ahead to act, close enough to be credible. Thirteen weeks is the common choice: it covers a quarter, and it reaches past most payment terms.
Reading it
The number that matters is the lowest projected balance, not the closing one. That trough is the point the business has to survive, and it is what determines whether a decision needs taking now.
A forecast built on a monthly average will misstate both ends of a seasonal year.