Jamieson, Fausset and Brown on Psalms 23:4

4. In the darkest and most trying hour God is near.

the valley of the shadow of death--is a ravine overhung by high precipitous cliffs, filled with dense forests, and well calculated to inspire dread to the timid, and afford a covert to beasts of prey. While expressive of any great danger or cause of terror, it does not exclude the greatest of all, to which it is most popularly applied, and which its terms suggest.

thy rod and thy staff--are symbols of a shepherd's office. By them he guides his sheep.