Need help with my gun movement script

So I have been following a tutorial series by ETeeski Tutorials, and I got stuck on fps 1.8. AKA scripting a basic gun with weight.

This is the script:

 #pragma strict
 
 var cameraObject : GameObject;
 
 var rotationSpeed : float = 0.3;
 
 @HideInInspector
 var targetXRotation : float;
 
 @HideInInspector
 var targetYRotation : float;
 
 @HideInInspector
 var targetXRotationV : float;
 
 @HideInInspector
 var targetYRotationV : float;
 
 var holdSide : float = 0.5;
 
 var holdHeight : float = -0.5;
 
 function Update () {
 
 transform.position = cameraObject.transform.position + Quaternion.Euler(0, targetYRotation, 0) * Vector3(holdSide, holdHeight, 0);
 
 targetXRotation = Mathf.SmoothDamp(targetXRotation, cameraObject.GetComponent(MouseLookScript).xRotation, targetXRotationV, rotationSpeed);
  
 targetYRotation = Mathf.SmoothDamp(targetYRotation, cameraObject.GetComponent(MouseLookScript).yRotation, targetYRotationV, rotationSpeed);
 
 transform.rotation = Quaternion.Euler(targetXRotation, targetYRotation, 0);
 }

I understand that the u set the position of the gun to follow the camera's position and then move it to a certain position using the Vector3. But if you are going to set the rotation of the Vector3 here, why use the variable "targetYRotation"? I don't really understand this part of the code and all help will be appreciated. Thanks!