I'm a newbie, begin with Tower Bridge Defence

Hi everyboby, I'm a newbie and I begin research Unity with Tower Birdge Defence project. Anyone same with me? We are together research this.
I have some problems, in prefab enemy1 (Assets->Prefabs->enemy1) have script Enemy and have:
void FixedUpdate ()
{
// Create an array of all the colliders in front of the enemy.
Collider2D[] frontHits = Physics2D.OverlapPointAll(frontCheck.position, 1);

// Check each of the colliders.
foreach(Collider2D c in frontHits)
{
// If any of the colliders is an Obstacle...
if(c.tag == "Obstacle")
{
// ... Flip the enemy and stop checking the other colliders.
Flip ();
break;
}
}

// Set the enemy's velocity to moveSpeed in the x direction.
rigidbody2D.velocity = new Vector2(transform.localScale.x * moveSpeed, rigidbody2D.velocity.y);

// If the enemy has one hit point left and has a damagedEnemy sprite...
if(HP == 1 damagedEnemy != null)
// ... set the sprite renderer's sprite to be the damagedEnemy sprite.
ren.sprite = damagedEnemy;

// If the enemy has zero or fewer hit points and isn't dead yet...
if(HP <= 0 !dead)
// ... call the death function.
Death ();
}

// Create an array of all the colliders in front of the enemy.
Collider2D[] frontHits = Physics2D.OverlapPointAll(frontCheck.position, 1);
Why is param second of OverlapPointAll fucntion is 1?
In docs, param second described is layerMask, I don't understand what is layerMask in this project, and why it equals 1?