unity basic c# script and how to use

this is technically day 1.5, it shoudlve worked day one but I only got it to work on day 2 the system collections but is probs useless but the tutorial i followed didn't remove it so idk.
suuper basic script but its got the important things, like 
using UnityEngine.SceneManagement; spelled correctly

and the fact that the part that says MainMenu is going to be the scripts title when u choose, so that public class part
yeah naming it is important

the SceneManager.LoadScene(SceneManager.GetActiveScene().buildIndex + 1);
also super important, it allows me / us to do the important thing, switch between  scenes, aka, what I'm going to do (maybe) levels :D

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

public class MainMenu : MonoBehaviour{

    public void PlayGame ()
    {
        SceneManager.LoadScene(SceneManager.GetActiveScene().buildIndex + 1);
    }
}