LayerMask allow you to display the LayerMask popup menu in the inspector.

Similar to those in the camera.cullingMask. Layer masks can be used selectively filter game objects for example when casting rays.

// Casts a ray using the layer mask,
// which can be modified in the inspector.
var mask : LayerMask = -1;
function Update () {
	if (Physics.Raycast (transform.position, transform.forward, 100, mask.value)) {
		Debug.Log("Hit something");
	}
}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    public LayerMask mask = -1;
    void Update() {
        if (Physics.Raycast(transform.position, transform.forward, 100, mask.value))
            Debug.Log("Hit something");
        
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	public mask as LayerMask = (-1)

	def Update() as void:
		if Physics.Raycast(transform.position, transform.forward, 100, mask.value):
			Debug.Log('Hit something')