The RF modules provided by cypress communicate in channels of bandiwth 1MHz, 98 such channels in the ISM band from 2.400 GHz to 2.498 GHz are used. RSSI gives the enviroment strength at that point of time. For more infomration on RSSI read through this link. RSSI is used by the transmitter to search for a quiter channel. If the channel has more noise the transmitter can jump in to another channel until it finds a quiet channel.
The typical RSSI value read in noise free channels is 20. The RSSI value can also be read at the receiver end. However this will not help in estimating the distance from the transmitter. Suppose the receiver and the transmitter are both on channel ' X '. Now the reciever reads RSSI on channel 'X', the RSSI value read is due to all the sources present in the channel along with transmitter. So it is difficult to distinguish the signal(sent by transmitter) from the noise(due to other sources on same channel). Hence the distance cannot be estimated based on RSSI value.