He will not suffer thy foot to be moved - He will enable you to stand firm. You are safe in his protection. Compare the notes at Psalms 38:16. This, with the remainder of the psalm, seems to be of the nature of an answer to the anxious question in Psalms 121:1 - an answer which the author of the psalm, in danger and trouble, makes to his own soul, imparting confidence to himself.
He that keepeth thee will not slumber - He will be ever watchful and wakeful. Compare Isaiah 27:3. All creatures, as far as we know, sleep; God never sleeps. Compare Psalms 139:11-12. His eyes are upon us by day, and in the darkness of the night - the night literally; and also the night of calamity, woe, and sorrow.