When shepherds were out in the field, they typically carried a rod and staff.
Your rod
Protection
One of the primary purposes of the shepherd’s rod was for protection. The rod was a branch with a knob on one end which was used like a club or to throw at a predator.
Considering how the shepherd used the rod, it seems that the rod may be symbolizing the Word of God in this psalm.
The Word has the ability to correctly judge and divide our thoughts and intentions of the heart, if we will bring them to it, and let it do its job.
The shepherd could also hurl the rod at a sheep at a much slower pace, to get the attention of the sheep that just wouldn’t listen and stay with the flock.
Rod as a symbol of authority
The rod was also the symbol of the Shepherd’s authority. It was also a symbol of Kingly authority. The rod meant that he was in control of the sheep and don’t you mess with them.
The rod as a tool
John 10:7-10 7 So Jesus said to them again, “Truly, truly, I say to you, I am the door of the sheep. 8 All who came before Me are thieves and robbers, but the sheep did not hear them. 9 I am the door; if anyone enters through Me, he will be saved, and will go in and out and find pasture. 10 The thief comes only to steal and kill and destroy; I came that they may have life, and have it abundantly.
When the sheep were being kept in the fields, a temporary sheepfold was often constructed using large rocks stacked one upon another in a large circle, forming an enclosure. The shepherd himself would often serve as the gate for this type of sheepfold.
There is only one way into the sheepfold. Jesus said in John 14:6 Jesus said to him, “I am the way, and the truth, and the life; no one comes to the Father but through Me.
And again in Acts 4:12 And there is salvation in no one else, for there is no other name under heaven given among men by which we must be saved.”
Accountability
Passing under the rod was part of a sheep’s daily life. As the sheep would enter the sheepfold, the shepherd would count them and make sure that all were present and accounted for. If he found any missing, he could block up the entrance to the sheepfold, and go and search for the missing sheep (Mathew 18:12-14). Jesus cares about each of us, and most especially when we are straying.
Inspection
The shepherd would use that pointy end of the rod to push back the wool of the sheep to inspect the sheep for wounds, infections, and insect infestations.
This process of passing under the rod meant that the sheep was cared for because the shepherd was looking after it.
David is comforted, because the Lord is looking after him personally. Psalm 139 v. 1-6 expresses David’s delight in the fact that the Lord knows who he really is, inside and loves him anyway.
We are valuable to God, and He will take care of us. He knows us intimately, and he loves us and values us anyway.
Shepherd’s Staff
A shepherd’s staff is perhaps the thing that identifies a shepherd more than anything else.
Draw close to our shepherd
First, notice that the thing that distinguishes a shepherd’s staff from other staffs is the crook of the staff. That kind of question-mark-shaped part at the top of the staff. This is used to draw the sheep closer to the shepherd.
And what a blessing it is, when the Lord draws us close. Psalm 65:4says How blessed is the one whom You choose and bring near to You To dwell in Your courts. We will be satisfied with the goodness of Your house, Your holy temple.
Draw close to one another
In this I think we see a picture of how the Holy Spirit draws us together as a Body of Christ
For gentle correction
The shepherd will frequently use the blunt end to jab the sheep and nudge it back in the direction of the flock.
Sometimes we get nudges or checks from the Holy Spirit if we start to wander off, or are headed in the wrong direction.
God uses these tools of His Word and the Holy Spirit to protect us, guide us, correct and draw us close to Him.