A figure of God's care of His people (De 32:10; Ps 17:8; Zec 2:8), and of the preciousness of the Divine law (Pr 7:2). In La 2:18 it is the source of tears.
C. W. Emmet.
"He found him in a desert land, And in the howling waste of a wilderness; He encircled him, He cared for him, He guarded him as the pupil of His eye.
Keep me as the apple of the eye; Hide me in the shadow of Your wings
Keep my commandments and live, And my teaching as the apple of your eye.
Their heart cried out to the Lord, "O wall of the daughter of Zion, Let your tears run down like a river day and night; Give yourself no relief, Let your eyes have no rest.
For thus says the LORD of hosts, "After glory He has sent me against the nations which plunder you, for he who touches you, touches the apple of His eye.