Unlike a profit and loss statement, which can show a business as profitable "on paper" even when it's short on actual cash, a cash flow forecast tracks the thing that actually determines whether a business can pay its bills: the real cash balance in the bank, month by month.
A basic forecast starts with the current cash balance, then adds expected monthly revenue and subtracts expected monthly costs for each future period, carrying the running balance forward. More detailed forecasts build in multiple scenarios — what happens to the cash position if revenue comes in 25% below plan, or above it — since actual results rarely match the single most-likely projection exactly.
For a startup or any cash-constrained business, a 12-month rolling cash flow forecast is one of the most important documents to keep current, because it's what turns "runway" and "burn rate" from abstract concepts into an actual month where, if nothing changes, the business runs out of money.