The usual formula is operating cash flow minus capital expenditure. The result is the cash that could be used to pay dividends, buy back shares, repay debt, make acquisitions or build up reserves. It is not the same as profit, because it ignores accounting items such as depreciation and focuses on actual cash movement.
Investors like it because it is harder to flatter than earnings. A company can report a profit and still have weak free cash flow if it spends heavily on equipment or if customers pay slowly.
It has limits too. It can swing from one quarter to the next with the timing of payments and large investments. A low or negative figure is not always bad, since a growing company may be spending on purpose to expand.
Look at free cash flow over several periods, not one, compare it with net income to see how well profit turns into cash, and read the reasons behind any big change.