。
She's gentle-hearted.
Literal
She [topic-は] heart [subject-が] gentle.
心が優しい follows the [abstract quality]が[adjective] pattern for describing inner qualities. 優しい (やさしい) means 'gentle, kind, tender' — it implies warmth and compassion more than deliberate helpfulness (which would be 親切). The が marks 心 as the subject of the adjectival predicate.