In the Bible the Lord is also called the Shepherd of the whole nation of Israel, for example in Psalm 28 verse 9, Psalm 80 verse 2.
In Psalm 23 verse 4 ‘the sheep’ addresses the Shepherd. This shows their personal relationship.
The 'passing through a valley' indicates a journey, a being on the way.
The sheep spend the summer in close proximity to the shepherd, who leads them to higher grassy mountain pastures. The best route there is always through the valleys. There are the least steep slopes, and there is also enough water and food for the journey.
However, in these valleys and canyons dangers of death also lurk: wild animals that can attack the sheep, sudden violent storms may turn mountain brooks into raging streams in a very short time. Loose rock and stone chips can fall down from the mountainside.
Then the shepherd is there to provide shelter in time.
‘Thy rod and Thy staff comfort me’
The rod or stick is a kind of club less than three feet long, with a leather strap attached, which the trained shepherd can throw with great accuracy at a predator or at a sheep that strays from the flock.
This is how the shepherd protects the flock.
The staff is longer and thinner than the rod, and has a curved end. It can serve to direct a sheep in the right direction, and with the curved end a sheep can be lifted out of a pit or gorge.
A sheep that searches for food with its head down all day long mainly orients by hearing. The tapping of the shepherd's staff on the ground indicates in which direction the shepherd is moving.
At the same time the sound of the staff gives the sheep a sense of the shepherd's proximity.
Rod and staff symbolize the Word of God; the Bible, in which He expresses His will and intention towards humanity. The rod denotes more the punitive aspect of the Word, the staff more of a light correction, but also direction in life, as well as security.
Psalm 119 verse 176:
I have gone astray like a lost sheep; seek your servant,
for I do not forget your commandments.
Mark 6 verse 34:
When He went ashore He saw a great crowd, and He had compassion on them, because they were like sheep without a shepherd. And He began to teach them many things.
In John 10 verse 11 Jesus says:
"I am the good shepherd. The good shepherd lays down his life for the sheep".
1 Peter 2 verse 25:
For you were straying like sheep, but have now returned to the Shepherd and Overseer of your souls.
In Psalm 23, David, the psalmist, writes that the Lord is his Shepherd, who cares for him, guides him, and provides in all physical and spiritual needs. This applies to him personally.