‘Keep me as the apple of the eye;
Hide me under the shadow of your wings,
From the unrighteous who oppress (spoil) me,
My deadly enemies, who encompass me about.'
The apple of the eye is the pupil. It represented the precious gift of sight. Thus it is above all things what a man guards, and it is protected by the eyelid. Thus the psalmist wants God to protect him as a man would protect his eyesight, indeed he wants him to be as an eyelid to him. The second illustration is that of the bird which takes its young under its wing for protection. Thus the psalmist claims dual protection.
The reason that he needs such protection is then given. The unrighteous, those who do not heed the voice of God, are oppressing him and seeking to despoil him. His deadly enemies are surrounding him, those who seek his death. They may have been internal enemies like Saul. They may have been external enemies. But the need is the same. The writer seeks protection from them all because he is YHWH's, because he is righteous and does observe YHWH's Law and YHWH's will.
Those whose trust is in God can look to God with confidence when unbelievers press in for He will be their eyelid to protect His precious eye, He will take them under His wings to protect His young.