While young, the little girl could play the guitar very well

这里省略了主语和谓语,还原就是
while she is young,the little girl could paly the guitar very well.
当while后面的be后的内容与girl形成修饰关系时,she is可以省略

while表示“当...的时候”，中间省略了主语，也就是“the little girl”,句子翻译就是，年轻的时候这个女孩就精通吉他了

while 有尽管的意思,相当于despite