。
She's a warm-hearted person, you know.
Literal
She [topic-は] heart [possessive-の] warm person [explanatory-なんです].
心の温かい (こころのあたたかい, 'warm-hearted') is a compound modifier: the adjective 温かい modifies the implied person via の. The sentence-final なんです (casual form of のです) adds an explanatory nuance — the speaker is explaining or asserting this as background context, as if answering an unspoken question. It's warmer and more emphatic than a plain です ending.