The RST System is used by Hams, and sometimes by other radio hobbyists, to report the quality and strength of a received signal. According to Wikipedia, "The code was developed in the 1934 by Amateur radio operator Arthur W. Braaten, W2BSR."
The complete code is a three digit number, with one digit each for conveying an assessment of the signal's Readability, Strength, and Tone. However, Tone is not meaningful for voice, so only two digits are used in voice reports.
Readability is a report on how easy is is to understand what is being received.
Strength is a report on how strong the received signal is.
Tone is a report on the quality of the tone of the CW characters or digital signal.
The codes are:
2--Barely readable, occasional words distinguishable.
3--Readable with considerable difficulty.
4--Readable with practically no difficulty.
1--Faint signals, barely perceptible.
2--Very weak signals.
5--Fairly good signals.
7--Moderately strong signals.
9--Extremely strong signals.
1--Sixty cycle a.c or less, very rough and broad.
2--Very rough a.c., very harsh and broad.
3--Rough a.c. tone, rectified but not filtered.
4--Rough note, some trace of filtering.
5--Filtered rectified a.c. but strongly ripple-modulated.
6--Filtered tone, definite trace of ripple modulation.
7--Near pure tone, trace of ripple modulation.
8--Near perfect tone, slight trace of modulation.
9--Perfect tone, no trace of ripple or modulation of any kind.
Occasionally, additional codes will be added to indicate other characteristics of the signal.
Some digital stations are adopting the RSQ system, which substitutes Signal Quality for Tone and redefines R and S in terms more applicable to digital signals. See http://www.rsq-info.net/ for more information.