Need explanation for this code

Hi all

Following is the code to fire bullet in the direction of mouse for a 2D platformer.(thanks Robertbu for the answer)

Can somebody please explain me the code from pos.z to var q? those three lines in below code? what exactly its doing

 var prefab : GameObject;
  
 function Update () {
 if (Input.GetMouseButtonDown(0)) {
 var pos = Input.mousePosition;
 
 pos.z = transform.position.z - Camera.main.transform.position.z;
 pos = Camera.main.ScreenToWorldPoint(pos);
 var q = Quaternion.FromToRotation(Vector3.up, pos - transform.position);
     
 var go = Instantiate(prefab, transform.position, q);
 go.rigidbody.AddForce(go.transform.up * 500.0);
 }
 }

Thanks