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

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.

Kenwood TS-520S S Meter the RST system

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

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