。
She sang the song gently.
Literal
She [topic-は] that song [object-を] gently sang.
やさしい has two distinct senses that run together in real usage: 優しい ('kind, gentle') and 易しい ('easy, simple'). The adverbial form やさしく works in both meanings, with context picking. Applied to singing, やさしく almost certainly means 'gently / softly / tenderly' rather than 'in an easy manner', though both readings are technically possible. The kanji 優しく resolves the ambiguity, but writers often leave it in kana when the meaning is clear from context. The continuative く form turns the i-adjective into an adverb describing the manner of singing.