Free cash flow is the number most valuation work ultimately cares about, because it is what owners can actually take out. It is harder to manipulate than net-income, though not impossible: stretching accounts-payable or capitalising costs both flatter it.
Definitions vary. Some subtract only capex; others also subtract acquisitions, leases or stock-based pay. State your definition, and use the same one on both sides of any comparison such as free-cash-flow-yield.
Example: Northwind Tools generates $164M of operating cash flow and spends $75M of capex, so free cash flow is $89M, 10.6% of revenue and $0.93 a share on 96 million shares.
Related: operating-cash-flow, capex, fcf-margin, free-cash-flow-yield, unlevered-free-cash-flow