Earnings yield turns the P/E into a percentage. A stock at a P/E of 20 has an earnings yield of 5%, which is the return you would get if all the earnings were paid out to you.
Joel Greenblatt's rule is to invest only when the earnings yield clearly beats the yield on the 10-year Treasury bond. The extra return you should demand for owning a stock instead of a safe government bond is about 3% to 5%, depending on how good the business is.
If the earnings yield is at or below the 10-year bond yield, you are not being paid for the extra risk.