Excuse me, what does the RE of RETSRA mean in aviation meteorological specification?

RE is the indicator code, which is used to compile the recent weather (if the following weather phenomena were observed from the last routine report or the past 1 h to this observation period, but no freezing precipitation, moderate or heavy precipitation, snow blowing, sandstorm or sandstorm was observed during this observation period; Thunderstorms, tornadoes, volcanic ash).

Make up at most three groups of recent weather. When you say TS, you mean thunderstorm, while RA means rain.