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
            }
        }

I would recommend checking the distance between your player object and the finish line object. Depending on the distance, you can gradually decrease the player character’s speed and load different animations.

Check the distance in your Update function, seperate of your triggerenter function. Should do the trick.