This is a metric used to get a view on whether a firm is over, or under, valued in cash flow terms.
It compares the annual free cash flow per share to the price per share as a percentage. So if FCPS is 10p and the share price is 100p, then the FCF yield is around 10%. The higher this is, the better value the share. Cash flow is thought by many analysts to be more reliable than earnings or even dividends, which is why this number is commonly quoted by analysts alongside the better-known P/E ratio and dividend yield.