The RST System (Readability - Strength - Tone)

From Wikipedia: 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. The code was developed in 1934 by Amateur radio operator Arthur W. Braaten, W2BSR, and was similar to that codified in the ITU Radio Regulations, Cairo, 1938.

The last element, "tone" is usually omitted from signal reports.

Link: https://en.wikipedia.org/wiki/R-S-T_system