The R-S-T system
The R-S-T system is used by amateur radio operators, shortwave listeners, and other radio hobbyists to exchange information about the quality of a radio signal being received. The code is a three digit number, with one digit each for conveying an assessment of the signal's Readability, Strength, and Tone.
For non-CW (non Morse code) signals, the Tone (i.e. last) digit is normally omitted.
Readability
R1 - Unreadable
R2 - Barely readable, occasional words distinguishable
R3 - Readable with considerable difficulty
R4 - Readable with practically no difficulty
R5 - Perfectly readable
Strength
S1 - Faint—signals barely perceptible
S2 - Very weak signals
S3 - Weak signals
S4 - Fair signals
S5 - Fairly good signals
S6 - Good signals
S7 - Moderately strong signals
S8 - Strong signals
S9 - Extremely strong signals
Tone
T1 - Extremely rough hissing note
T2 - Very rough AC note, no trace of musicality
T3 - Rough, low-pitched AC note, slightly musical
T4 - Rather rough AC note, moderately musical
T5 - Musically modulated note
T6 - Modulated note, slight trace of whistle
T7 - Near DC note, smooth ripple
T8 - Good DC note, just a trace of ripple
T9 - Purest DC note