Psalms 17:8 Keep me as the apple of the eye, hide me under the shadow of thy wings,
Proverbs 7:2 Keep my commandments, and live; and my law as the apple of thine eye.
It is an old English use of the word "apple" and does not refer to the fruit we call the "apple" today. It refers to the center of the pupil of the eye.
When you make something the "apple of your eye", it means to watch over it and protect and cherish it.
photos c/o: kristen rae photography
skirt: forever 21