The RST System is used as a standard for exchanging signal reports between amateur radio stations and represents the quality of the signal.

Readability

1 Unreadable
2 Barely readable, occasional words distinguishable
3 Readable with considerable difficulty
4 Readable with practically no difficulty
5 Perfectly readable

Readability is used in the RST system to convey how easy the signal is to comprehend. A strong signal with good audio is usually referred to as a 59.

Signal strength

1 Faint signals, barely perceptible
2 Very weak signals
3 Weak signals
4 Fair signals
5 Fairly good signals
6 Good signals
7 Moderately strong signals
8 Strong signals
9 Extremely strong signals

Kenwood TS-520S S Meter the RST system

The S meter on a Kenwood TS-520S transceiver, circa 1980.

Tone

1 Fifty cycle AC or less, very rough and broad
2 Very rough AC, very harsh and broad
3 Rough AC tone, rectified but not filtered
4 Rough note, some trace of filtering
5 Filtered rectified AC 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

For more details, visit R-S-T system on wikipedia.