The expression would normally only use the singular - 'I am the apple of his eye'. It means you are very special to him, someone he only thinks good things about.

It is originally a biblical term. A few possible meanings are: to be cherished above all others, to be very dear to someone, or to be the focus of someone (since the pupil is the center of the eye and focuses on objects).

Thanks a lot, Brittney!! ;)