what is sleep mode?

hard ware used: xbee series 2 modem type:xb24-B version: 1041 znet 2.5

what is sleep mode? what is the signifiance of cyclic sleep? how to establish communication between two xbee module in sleep mode without data lose? is there any configuration setting? what are all ther paramenter are to be set, kindly mail me as soon as possibel