“You will not be afraid of the terror of night, Nor of the arrow that flies by day.”
In ancient times the enemy would attack villagers at night while they were sleeping.
God is saying you can sleep peacefully. He stands watch for your safety. Arrows are any incidence in life, which are from the enemy.
God makes sure it will not reach nor touch you.
Again this chapter is to teach us how in detail God is in protecting His children.