It is often used to describe a child or romantic partner, but it can also be used for anything else that is especially dear to you.
Here are some examples of how it can be used:
* "Her grandson is the apple of her eye." (Her grandson is her most cherished person.)
* "That vintage car is the apple of his eye." (He loves that car more than anything.)
* "The new puppy is the apple of their eyes." (They adore their new puppy.)
Origin of the idiom:
The phrase comes from a passage in the Bible (Deuteronomy 32:10), where God is described as protecting Israel "as the apple of his eye." This refers to the pupil of the eye, which is very delicate and needs protection.
Other idioms with similar meaning:
* "The light of your life"
* "Your pride and joy"
* "Your everything"