TetherExample.cs

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

public class TetherExample : MonoBehaviour
{
    public Transform anchor;
    public Transform player;
    public LineRenderer lineRenderer;

    private Vector3 anchorVelocity;

    private void Update()
    {
        MoveAnchor();
        MovePlayer();
        ConstrainPlayer();
        DrawLine();
    }

    private void MovePlayer()
    {
        if (Input.GetKey(KeyCode.W)) {
            player.position += Vector3.up * 0.25f;
        }
        if (Input.GetKey(KeyCode.A)) {
            player.position += Vector3.left * 0.25f;
        }
        if (Input.GetKey(KeyCode.S)) {
            player.position += Vector3.down * 0.25f;
        }
        if (Input.GetKey(KeyCode.D)) {
            player.position += Vector3.right * 0.25f;
        }
    }

    private void MoveAnchor() 
    {
        //move the anchor a random amount
        anchorVelocity += new Vector3(Random.Range(-0.01f, 0.01f), Random.Range(-0.01f, 0.01f), 0);

        //make sure the anchor's velocity doesn't get too high
        if (anchorVelocity.magnitude > 0.2f) {
            anchorVelocity = anchorVelocity.normalized * 0.2f;
        }

        anchor.position += anchorVelocity;
    }

    private void ConstrainPlayer()
    {
        float maxDistance = 3;
        if (Vector3.Distance(anchor.position, player.position) > maxDistance) {
            Vector3 anchorToPlayer = player.position - anchor.position;
            anchorToPlayer = anchorToPlayer.normalized * maxDistance;
            player.position = anchor.position + anchorToPlayer;
        }
    }

    private void DrawLine()
    {
        lineRenderer.SetPosition(0, player.position);
        lineRenderer.SetPosition(1, anchor.position);
    }
}