taru-adjective, adverb taking the 'to' particle
clear and resonant; sonorous; full-sounding
Describes a voice, singing, or musical sound that is clear, bright, and carries well. Often used with と as an adverb.
彼の朗々とした声がホールに響いた。
His clear, resonant voice echoed through the hall.
彼女は朗々と歌い上げた。
She sang out in a full, sonorous voice.
Means 'clear' in a general sense (e.g., water, sky, voice), but lacks the resonant, full-bodied nuance of 朗々.
The exact derivation is uncertain. The kanji 朗 means 'bright' or 'clear', and the reduplication 朗々 intensifies the sense of clarity and resonance.