After preaching three sermons in the preceding chapters of Deuteronomy, Moses changed the form of his message to singing. Sometimes reciting something in a different form makes it easier to remember.
This song gives a brief history of Israel. It reminds the people of their mistakes, warns them to avoid repetition of those mistakes, and offers the hope that comes only in trusting God.
As a result, the Israelites had no excuse for abandoning God. He had shielded them like a kindly shepherd. He had guarded them like a person protects the pupil of his eye.
For the people, God had been the encircling protector, like a mother eagle who protects her young. The Lord alone had led them.
Today, the Lord God alone leads us. Let us remember to trust in him. Let us remember He is our protector and He guards us like the apple of His eye.