My Character controller keeps jumping infinitely

I have followed an old unfinished tutorial to try and learn C# and I got the character controller working but when i jump my character controller infinitely jumping. i have tried multiple things but nothing has worked. i'm not asking for people to do it for me, i want an explanation on how to make it work properly.

thanks in advance.

here is my code:

using System.Collections; using System.Collections.Generic; using UnityEngine;

public class PlayerMov : MonoBehaviour {

 public Transform playertrn;
 public Rigidbody playerRB;
 public int speed;
 private bool isFalling = false;
 private bool doubleJump = false;
 public Vector3 JumpHeight = new Vector3 (0, 8, 0);


 void Start () 
 {
     playertrn = GetComponent<Transform> ();
     playerRB = GetComponent<Rigidbody> ();
     Physics.gravity = new Vector3(0, -15.0F, 0);
 }
 

 void Update () 
 {
     if (Input.GetKey (KeyCode.D)) 
     {    
         playertrn.Translate (new Vector3(speed, 0, 0) * speed * Time.deltaTime);
     }

     if (Input.GetKey (KeyCode.A)) 
     {
         playertrn.Translate (new Vector3 (-speed, 0, 0) * speed * Time.deltaTime);
     }

     if (Input.GetKey (KeyCode.Space)) 
     {
         playerRB.velocity = JumpHeight;
         isFalling = true;
     }

} void OnCollisionStay () { isFalling = false; } }