Why An S meter is a receiver indicator calibrated to show the relative strength of the incoming signal, normally driven from the receiver's AGC voltage. The scale runs S1 through S9, and above S9 it is marked in decibels over S9. By convention on HF, S9 corresponds to 50 microvolts at the antenna terminals into 50 ohms, and one S unit is nominally 6 dB, so a change of one S unit means about a four-fold change in received power.
Watch out Transmitter power output is read on a wattmeter or power meter, not an S meter, which is a receiving-side instrument; impedance needs an antenna analyzer or bridge, and carrier suppression is a transmitter SSB spec measured on a spectrum analyzer.
S is for Signal strength, on the receive side. S9 = 50 microvolts, 1 S unit = 6 dB.