After his last health scare, he decided to turn over a new leaf and quit smoking.

turn 是动词，本义是"转动、翻转"。 over 在这里是副词，与 turn 组成 turn over，意思是"翻转"。 turn over a new leaf 是一个固定短语，字面意思是"翻开新的一页"——leaf 指书页，借此比喻"改过自新、重新开始"。