Free cash flow is the cash the business generated after paying to maintain and grow its asset base. Unlike net income it is difficult to shape with accounting judgement, which is why many analysts treat it as the more honest number.
It is what actually funds shareholder returns. A dividend paid out of borrowing rather than out of free cash flow can continue for years, but it is a decision about the balance sheet rather than a distribution of profit, and it eventually ends.
Profit and cash flow diverge for legitimate reasons as well as suspicious ones. A fast-growing company funding inventory and receivables can be profitable and cash-negative for a long stretch. A mature company where the gap widens every year with no such explanation is a different situation.