LayerMask Explained

Use NameToLayer() to get a layer's index.
It is used to set or compare an object's layer.

int layerIndex = LayerMask.NameToLayer("Default");
bool sameLayer = gameObject.layer == layerIndex;
gameObject.layer = layerIndex;

The other methods in Unity, such as Raycast(), require a LayerMask.
A LayerMask contains a 1 or 0 for all 32 layers.

Use GetMask() to create a LayerMask which includes all the layers specified.
Or use it to create masks with single layers, and use the OR | operator to create combinations between them.

LayerMask a = LayerMask.GetMask("Default");
LayerMask b = LayerMask.GetMask("Water");
LayerMask c = LayerMask.GetMask("Default", "Water");

LayerMask raycastLayers = a | b;  // The same as 'c'

Raycast script example

using UnityEngine;

public class LayerMaskScriptExample : MonoBehaviour
{
    public float distance;
    public LayerMask raycastLayers;

    private void Awake()
    {
        LayerMask a = LayerMask.GetMask("Default");
        LayerMask b = LayerMask.GetMask("Water");
        LayerMask c = LayerMask.GetMask("Default", "Water");

        raycastLayers = a + b;  // The same as 'c'
    }

    private void Update()
    {
        if (Physics.Raycast(transform.position, transform.forward, distance, raycastLayers))
        {
            Debug.Log("Hit something on a layer included in the mask!");
        }
    }
}

That is all which is needed to work with LayerMask in Unity.
Further reading is optional.

LayerMask Functions

Advanced (BitMask, Bit Shifting)

LayerMask Utilities