Received signal strength indication (RSSI)

       In telecommunications, received signal strength indicator (RSSI) is a measurement of the power present in a received radio signal.

      RSSI is a generic radio receiver technology metric, which is usually invisible to the user of the device containing the receiver, but is directly known to users of wireless networking of IEEE 802.11 protocol family.

     In an IEEE 802.11 system RSSI is the relative received signal strength in a wireless environment, in arbitrary units. RSSI is an indication of the power level being received by the antenna. Therefore, the higher the RSSI number (or less negative in some devices), the stronger the signal.

Reference : http://en.wikipedia.org/wiki/Received_signal_strength_indication