taru-adjective, adverb taking the 'to' particle
Literary taru-adjective describing a vigorous, surging energy or ambition. Often used in written descriptions of enthusiasm, vitality, or burgeoning force.
彼の勃勃たる野心は誰にも止められなかった。
His surging ambition could not be stopped by anyone.
若者たちの勃勃とした活力が会場に満ちていた。
The venue was filled with the spirited energy of the young people.
旺盛 is a more common na-adjective meaning vigorous or abundant, used in everyday contexts, while 勃勃 is literary and emphasizes a rising, surging quality.
The exact derivation is uncertain; the kanji 勃 means 'sudden rise' or 'vigor', and the reduplicated form 勃勃 is a literary Chinese-derived pattern used to intensify the meaning.