What is cash flow?
Cash flow is different from revenues and expenses. Revenues and expenses are reported on your income statement as they are incurred, not as they are received or paid. They are not yet cash in the bank.
Cash flow, on the other hand, is the actual "cash" that is received or paid during the course of operating your business. Though it's called "cash", it can originally be received or paid in the form of checks, email money transfers, PayPal payments, credit card payments, etc. Once honoured, these amounts become cash flow.
There is an old saying in business that "cash is king". That's because healthy cash flow is vital to operating a healthy business.