The formula
Free Cash Flow = Operating Cash Flow − Capital Expenditure. It's the cash left over after the company has paid to run and maintain/grow its asset base.
How to read it
FCF is the real cash a business can use to pay down debt, pay dividends, or reinvest. It's harder to manipulate than reported profit, which is why consistently positive and growing FCF is one of the strongest signs of a healthy business.
Profit up but FCF down, quarter after quarter, is a classic warning that earnings aren't being backed by real cash.