She'd left her gloves at home, so she had to keep her hands in her pockets to stay warm.

She'd 是 She would 或 She had 的缩写形式，在这里是 She had 的缩写，表示过去完成时，意思是"她已经"。 left 是动词 leave 的过去分词形式，leave 的意思是"离开、留下、遗忘"。 She'd left 表示"她已经忘记/落下"的意思。