What an S-unit is
The S-meter on a receiver measures signal strength on a scale from S1 to S9, then in decibels over S9. The IARU Region 1 recommendation, also used by most manufacturers as a target, defines S9 as −73 dBm on HF, which is 50 µV across 50 Ω, and each S-unit as 6 dB, so each step is double the voltage or four times the power. Above 30 MHz, S9 is −93 dBm, or 5 µV, because signals there are usually weaker and the background noise is lower.
Real meters aren't accurate
Very few receivers follow the standard closely. Many radios read S9 with less signal than 50 µV, have S-units of 3 to 5 dB rather than 6, and change their calibration with the preamp or attenuator switched in. The figures here are what the standard says, so use them to compare reports or plan a link, and calibrate your own meter with a signal generator and step attenuator if you need real numbers.
Signal reports
An S-reading is the S in an RST report, such as 59. Because the band's noise level often reads S3 to S7 on its own, a signal at S5 on a quiet band can be easier to copy than S8 on a noisy one. The RST scale explains the rest of the report, and the link budget calculator predicts the S-reading at the other end of a path.
Questions
What is S9 in dBm?
−73 dBm on HF, which is 50 µV into 50 Ω. On VHF and above it's −93 dBm, or 5 µV.
How many dB is one S-unit?
6 dB in the standard, which is twice the voltage or four times the power. Many real meters use fewer dB per unit.
What does S9+20 mean?
A signal 20 dB stronger than S9: −53 dBm, or 500 µV, on HF.
How do I convert dBm to dBµV?
In a 50 Ω system, add 107: −73 dBm is 34 dBµV.