The "R" stands for "Readability" and is measured on a scale of 1, (barely readable) to 5 (Perfectly readable).
The "S" stands for "Strength" and is measured on a scale of 1, (very weak signal) to 9 (very strong signal).
The "T" stands for "Tone" and is measured on a scale of 1, (very poor tone) to 9 (perfect tone). Tone is used only used in morse code and digimode transmissions and is therefore omitted during voice operations.
An example RST report for a voice contact would be "59", usually pronounced "five and nine," and symbolises a perfectly readable and very strong signal. Exceptionally strong signals are designated by the quantitative number of decibels, in excess of "S9", displayed on the receiver's "S-meter". Example: "Your signal is 30db over S9."