Description

Returns the identity matrix (Read Only).

This is a matrix that effectively does nothing when applied. It has 1s in the main diagonal and 0s in all other elements:

using UnityEngine;

public class ExampleScript : MonoBehaviour
{
    void Start()
    {
        // will print:
        // 1.00000 0.00000 0.00000 0.00000
        // 0.00000 1.00000 0.00000 0.00000
        // 0.00000 0.00000 1.00000 0.00000
        // 0.00000 0.00000 0.00000 1.00000
        var matrix = Matrix4x4.identity;
        Debug.Log(matrix);
    }
}

Additional resources: Matrix4x4.zero, isIdentity.