Young, Juicy, Lively, Vibrant
い adjective
みずみずしい
彼女の瑞々しい歌声が人々を魅了した。
Everyone was fascinated by her vibrant voice.
You give your congratulations to someone a few times because they seem so fresh and vibrant. You feel like you should because it must be nice to be like that still. Someday it will be gone, so make sure you say it a few times right away.
The reading is just the kun'yomi reading (which you learned with the kanji) twice in a row.