What does Psalm 121:4 mean?

Continuing the encouragement of the prior verse (Psalm 121:3), the second voice of this psalm assures the first speaker that God is never caught unaware. God is perfectly aware of all that happens or will happen. At all times He watches over His people and is their Guardian.

The Hebrew word translated "keep" comes from the same word used in Genesis 2:15, when the Lord took Adam and put him in the garden of Eden. It was Adam's responsibility to take good care of the garden: to "keep" it. Similarly, the Lord promised to take good care of Jacob and his descendants, the people of Israel. Genesis 28:15 reports the Lord's promise: "Behold, I am with you and will keep you wherever you go." History is replete with evidence of the Lord's preservation of Israel. From Israel's escape from slavery in Egypt until today, Israel has survived numerous efforts to annihilate the race, but Israel has always emerged.