what is a Projectile?

i’m follwing the video of ‘youtube’.

this code is a following codes. but Projectile is red underline…
so, i can not proceed this video lecture…
would you please answer my question?
i’d like to proceed the lecture…T.T

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

public class Gun : MonoBehaviour {

public Transform muzzle;
public Projectile projectile;
public float msBetweenShots = 100;
public float muzzleVelocity = 35;
public void Shoot()
    Projector newProjector = Instantiate(projector, muzzle.position, muzzle.rotation) as Projector;


Projectile is a custom type, most likely made by the guy who did the video.
It could be anything, for example:

public class Projectile

it also could be a struct:

public struct Projectile

It could be a class deriving from MonoBehaviour (which is the most likely scenario, because the projectile is public in the code you show and therefore probably supposed to be drag&dropped in the inspector of the gun class).

public class Projectile : MonoBehavior