p/e ratio is equal to Market price divided by earnings per share. if 2 company's p/e ratio are compared, a company with lesser p/e considered as under priced. there is nothing like good p/e ratio. lesser the p/e ratio, it is under priced.
in a bear phase, many stocks may be available at a p/e ratio of less than 5 also. whereas in bull phase, it may not be available