Free cash flow (FCF) is the profit measure value investors care about most. It is the cash left over after the business has paid all its operating costs and spent what it needs on equipment and facilities to keep running and to grow.
This is the cash that can pay dividends, buy back shares, pay down debt, or buy other companies. In the end, it is the cash that belongs to shareholders.
One thing to keep in mind: the money a company spends on long-term assets (capital expenditure, or capex) comes in two flavors. Maintenance capex keeps the business running as is, and growth capex expands it. For a quality business, maintenance capex should be only a small part of operating cash flow.