how do i make a check point and a death barrier

Questions & Answers
1

im making a simple game were you move a ball from platform to platform by balancing on beams so how would i make it so when you reach a platform you check point and when you fall off you respawn at that check point

2

Something like this might work (not tested).

CheckPoint.cs

public class CheckPoint : MonoBehaviour
{
    public static CheckPoint LastCheckPoint
    {
        get;
        private set;
    }

    private void OnTriggerEnter(Collider other)
    {
        LastCheckPoint = this;
        gameObject.SetActive(false);
    }
}

DeathCheck.cs

public class DeathCheck : MonoBehaviour
{
    public float deathBarrier = 0.0f;

    private void Update()
    {
        if(this.transform.position.y > this.deathBarrier)
        {
            var lastCheckPoint = CheckPoint.LastCheckPoint;
            if(lastCheckPoint != null)
            {
                this.transform.position = lastCheckPoint.transform.position;
            }
            else
            {
                Debug.LogError("Player is dead but have not reach a check point.");
            }
        }
    }
}

Attach CheckPoint.cs and a trigger enabled collider to your checkpoints.

Attach DeathCheck.cs to the player.

Edit: added a simpler death check script

SimpleDeathCheck.cs

using UnityEngine;

/// <summary>
/// Component to put player at start position of player gets below the death barrier.
/// </summary>
public class SimpleDeathCheck : MonoBehaviour
{
    /// <summary>
    /// The minimum Y position the player is allowed to have before being moved back to start.
    /// </summary>
    [SerializeField]
    private float deathBarrier = -10.0f;

    /// <summary>
    /// The position to apply to the player if player gets below the death barrier.
    /// </summary>
    [SerializeField]
    private Vector3 startPosition = new Vector3(0.0f, 0.5f, 0.0f);

    /// <summary>
    /// Checks if player is below death barrier and repositions the player back to start if so.
    /// </summary>
    private void Update()
    {
        if (this.transform.position.y < this.deathBarrier)
        {
            this.transform.position = this.startPosition;
        }
    }
}