。
She's kindhearted.
Literal
She [topic-は] heart [subject-が] gentle.
やさしい is the hiragana writing of 優しい ('gentle, kind'), and 心がやさしい thus describes someone as 'gentle-hearted, kindhearted.' Japanese writers often have a choice between hiragana and kanji for emotive adjectives like this — the hiragana form feels slightly softer and more conversational, while the kanji form 優しい reads as more formal or literary. This kind of script selection is a subtle stylistic decision in Japanese writing.