Albert Barnes Commentary Psalms 91:4

"He will cover thee with his pinions, And under his wings shalt thou take refuge: His truth is a shield and a buckler." — Psalms 91:4 (ASV)

He shall cover you with his feathers ... - As the parent bird protects its young. See the notes at Psalm 17:8. Compare Deuteronomy 32:11. His truth. His unfailing promise; the certainty that what he has promised to do he will perform.

Shall be your shield and buckler - literally, Shield and buckler is his truth. The meaning is, that his pledge or promise would be to them as the shield of the soldier is to him in battle. Compare Psalm 35:2. The word rendered “buckler” is derived from the verb “to surround,” and is given to the defensive armor here referred to, because it “surrounds,” and thus “protects” a person. It may apply to a coat of mail.