Newbie GNUradio: displaying received Signal RSSI

Hello Experts,
first of all I would like to know what RSSI actually is and if it is
possible to calculate/display the Received Signal RSSI using USRP block
in GNUradio over Ubuntu 13.10.

2nd: I have connected the waterfall plot directly to the output of
Receiver USRP block but it is showing blue screen all the time. (am I
doing it wrong, do i need something else in between for conversion etc).

I shall be very much thankful for your guidance.

Best Regards
Ali