When a radio operator makes contact with another station the most basic exchanges that are made are Callsign, Operator name, Signal strength and voice quality. For the last two we use the R-S-T system.
R - Readability
S - Signal Strength
T - Tone
Within this system the R will indicate how well the other station can understand the transmitted audio. This ranges on a scale of 1-5, with 1 being 'unreadable' and 5 being 'perfectly readable'. The are many factors that can affect the readability, atmospheric noise, static crashes, signal fading, and man made noise being some of them.
The S in the report is the strength of the signal. This is important to the transmitting station as it will give a clear understanding of how well the operator has set up their 'shack'. Is the antenna tuned and working correctly, is the coax performing as it should and is the radio transmitting at the power that is expected. The signal strength is 1 - 9 with 1 being a 'faint signals, barely perceptible', and a signal strength of 9 is an 'extremely strong signal.'
The third number refers to Tone and applies to CW (Morse Code).
Below is a table with the respective numbers