How to stop player movement slowly fade down?

Hi, sorry i'm Unity beginner and i'm frustated with my current problem. Well i build a running game that player will run until finish line. Near the finish line i want my player stop slowly, i mean the speed movement slowly reduce until its completely stop. how to do that? i have looking for it around this page but i did't get what i need. I try using AddForce but i didn't understand what it is. So, anybody know the simple easy way to do my animation? i move my player with this code example :

  if (Input.GetMouseButtonDown(0) && flagStart==0 && counter==0)
         {
             dir = Vector3.forward; flagStart = 1;
         }

and here what should i do to my code :

 void OnTriggerEnter(Collider other)
         {
             if (other.tag == "Player")
             {
                  //code to stop slowly
             }
         }