This common phrase is often used to identify an object of one's special favor or affection. The apple of the eye, of course, is not a fruit, but the pupil of the eye, so essential for sight that it becomes a peculiarly apt symbol for a prized possession. It is used five times in the Bible as a translation of three different Hebrew nouns, none of which refer to the actual apple fruit. In each case, however, it speaks of something highly valuable to the owner.
Three of these (Deuteronomy 32:10; Lamentations 2:18; and our text above) are in reference to the chosen people, Israel, as the "apple of the eye" of God Himself. God has often punished Israel for her sins and has allowed other nations to be His rod of judgment, but woe to that nation that touches the apple of His eye in this way!
That individual believers can also be so regarded by the Lord is evident from one of David's prayers: "Shew thy marvellous lovingkindness. . . . Keep me as the apple of the eye, hide me under the shadow of thy wings" (Psalm 17:7-8).
To be kept by God as He would keep the very apple of His own eye, requires an implicit trust in Him and His Word. In fact, His Word must become the apple of our eye! "My son, keep my words, and lay up my commandments with thee. Keep my commandments, and live; and my law as the apple of thine eye" (Proverbs 7:1-2).
There are many beautiful and appropriate figures used for God's Word ("light," "hammer," "sword," "milk," etc.), but none more personally meaningful than this. May the Holy Scriptures, the indispensable Word of God, truly be the apple of the eye for each of us! HMM