taru-adjective, adverb taking the 'to' particle
Literary 'taru' adjective describing a joyful or cheerful manner. Often used adverbially with と (欣然と) to mean 'gladly' or 'with delight'. Rare in modern conversation; mainly found in written or formal contexts.
彼はその知らせを聞いて欣然と引き受けた。
He gladly accepted upon hearing the news.
老人は欣然として若者に道を教えた。
The old man cheerfully gave directions to the young person.
The kanji 欣 means 'rejoice' or 'take pleasure in', and 然 is a suffix forming adjectives/adverbs describing a state. The reading きんぜん is the on'yomi. The variant 忻然 uses a rare character 忻 with a similar meaning.