New to Unity/C#, I've got a few questions on a tutorial.

I'm doing the tutorial on Survival Shooter. I'm following the videos and the PDF that has changes for Unity 5. In part 7 there are some pre-programmed scripts. The Shoot() script is below, which is when the player shoots the gun. I have a couple questions if anyone can please answer for me.

1. Why is the layermask, which is shootableMask, stored as an integer, and what do the arguments in the Physics.Raycast method actually mean? Raycasting in general is just confusing for me.

2. You will notice that right after gunLine which is the Line Renderer, is enabled, its position is set to (0, transform.position). But towards the bottom, the code reads: gunLine.SetPosition (1, shootHit.point); The shootHit.point I can tell basically means where the bullets are actually going to land, but I don't understand where the 1 came from. In the tutorial it was described as the "second point" but that just confused me more.

3. After the raycasting is done, there is this line of code: EnemyHealth enemyHealth = shootHit.collider.GetComponent <EnemyHealth> ();

shootHit was declared as a RaycastHit. No idea how this is a reference to the script EnemyHealth, which is also in the game. If someone could break down what's happening in this line of code, it would be great.

void Shoot ()
{
    timer = 0f;

    gunAudio.Play ();

    gunLight.enabled = true;

    gunParticles.Stop ();
    gunParticles.Play ();

    gunLine.enabled = true;
    gunLine.SetPosition (0, transform.position);

    shootRay.origin = transform.position;
    shootRay.direction = transform.forward;

    if(Physics.Raycast (shootRay, out shootHit, range, shootableMask))
    {
        EnemyHealth enemyHealth = shootHit.collider.GetComponent <EnemyHealth> ();
        if(enemyHealth != null)
        {
            enemyHealth.TakeDamage (damagePerShot, shootHit.point);
        }
        gunLine.SetPosition (1, shootHit.point);
    }
    else
    {
        gunLine.SetPosition (1, shootRay.origin + shootRay.direction * range);
    }
}

}

Any help would be greatly appreciated.