Free cash flow is the cash a company's operations generated after paying for the physical assets it needs to keep running and growing. It is the cash available to repay debt, pay dividends, buy back shares or make acquisitions.
Formula
free cash flow = operating cash flow − capital expenditure
Both inputs are as reported: operating cash flow and capital expenditure for the same period. Capital expenditure is a positive amount spent, so it is subtracted. TTM free cash flow is the sum of the last four quarters.
Why it may differ from the company's figure
Free cash flow is not defined by accounting standards. When a company quotes it in an earnings release, it is a non-GAAP measure and the company chooses the definition: some subtract capitalized software, lease payments or the cash cost of acquisitions; some add items back. Listed uses the one formula above for every company, so figures are comparable across companies, but they will not always match what a company says.
Reading it
Negative free cash flow is not a verdict on its own. A young company investing heavily, or a mature one in a large build-out year, can have negative free cash flow for a while. Look at several years and at what the cash was spent on.
When it says "not reported"
Either input is not reported for the period.