To be "the apple of one's eye," means that the subject is cherished or valued above other things. It is usually used to refer to a person, typically in a parent-child context. For example:
"the apple of one's eye," 一般指的是人,而且一般是家长对孩子的。它的意思是说家长(或老师)很珍惜,很爱这个孩子或徒弟。比如:
His oldest daughter is the apple of his eye.
他珍爱他的大女儿。