YouTube can't help me Error

So im trying to learn Unity and C# by just doing it with this youtubes video How to make a Video Game in Unity - PROGRAMMING (E02) - YouTube
However I ran into a small problem that made my game pause ever time I hit play looking stuff online I found it was the error pasue function and truned it off and started working fine. I still have the error and simply want to know what I did wrong. Im using Unity 5.5 and this is my code. Please help me learn.

using UnityEngine;

public class NewBehaviourScript : MonoBehaviour
    {
    public Rigidbody rb;
	// Use this for initialization
	void Start ()
    {
        rb.AddForce(0, 200, 500);
    }
	
	// Update is called once per frame
	void Update ()
    {
       
    }
}

Have you set rb variable in Inspector?

I dont even know how to check that