Definition - What does Signal Strength mean?
Signal strength, in telecommunications, is the magnitude of an electric field at a reference point, which is located at a significant distance from the transmitting antenna. This is expressed in terms of the signal power of the receiver or the voltage per length recieved by the reference antenna.
Techopedia explains Signal Strength
High-power transmissions used in broadcasting are expressed in dB-millivolts per meter (dBmV/m). In low-power systems, including mobile phones, the signal strength is expressed in dB-microvolts per meter (dBµV/m).