In short: Cash flow is the movement of money over a period — how much comes in, how much goes out, how much remains, and exactly when.
Simply put: It matters not only how much but when: pay arrives once a month, bills whenever they like. The accounting standard IAS 7 splits cash flow into operating (everyday life), investing (buying or selling assets) and financing (taking or repaying loans). Profit is not cash: a business or project can make a profit on paper and still run out of money if payments come late.
Example: €1,600 comes in and €1,450 goes out — the monthly flow is +€150. If, without income, you spend a net €1,000 a month and have a €3,000 reserve, the money lasts 3 months.
Watch out: Myth: "If there is profit, there will be cash." Not necessarily. Check what is left every week, not at the end of the year — money runs out on a particular day.
Source: IFRS, IAS 7 Statement of Cash Flows — https://www.ifrs.org/issued-standards/list-of-standards/ias-7-statement-of-cash-flows/