A business can be profitable on paper and still run short of cash. Cash flow measures what actually enters and leaves the bank account, and when.
Track expected inflows
Start with open client invoices and their likely payment dates. Invoice date and payment date are not the same thing.
Use each client's real payment behavior in your forecast.
Plan fixed and variable outflows
Rent, payroll, taxes, insurance, software and debt payments should be visible before they hit the account.
Then add project costs, materials and subcontractors. That is often where gaps appear.