。
She is very studious.
Literal
She [topic-は] very study-zealous is.
勉強熱心 is a compound stacking 勉強 (study) with 熱心 (zealous, earnest) — the second member shapes the whole into a na-adjective meaning 'studious.' Compounds with 熱心 are productive: 仕事熱心 (work-zealous, hardworking), 教育熱心 (zealous about education). Each one names a domain you throw yourself into.