。
She's a naturally kindhearted person.
Literal
She [topic-は] innate [possessive-の] heart [subject-が] gentle person is.
心の優しい人 means 'a kindhearted person' — literally 'a person whose heart is gentle.' This uses the relative clause structure where 心が優しい (heart is gentle) modifies 人. 生まれつきの makes this attributive: 'innate/natural.' The の after 生まれつき allows it to modify the following noun phrase directly.