What is the #region directive used for?

public class MyClass
{
    #region Cool Stuff

    #region Cool Constants

    /// <summary>
    /// The minimum Cool value.
    /// </summary>
    public const int CoolMin = 0;
    /// <summary>
    /// The maximum cool value.
    /// </summary>
    public const int CoolMax = 100;

    #endregion

    /// <summary>
    /// Raised when Cool changes.
    /// </summary>
    public event EventHandler CoolChanged;

    /// <summary>
    /// The internal value of Cool.
    /// </summary>
    private int cool;

    /// <summary>
    /// Gets or sets Cool.
    /// </summary>
    public int Cool
    {
        get { return cool; }
        set
        {
            if (cool != value)
            {
                if (CoolValidation(value))
                {
                    cool = value;
                    OnCoolChanged(EventArgs.Empty);
                }
                else
                    throw new ArgumentOutOfRangeException(
                        "Cool",
                        string.Format("Cool must be between {0} and {1} inclusive", CoolMin, CoolMax));
            }
        }
    }

    #region Cool Methods

    /// <summary>
    /// Checks whether the specified value is between CoolMin and CoolMax.
    /// </summary>
    /// <param name="cool">The value to test.</param>
    /// <returns>true if cool is between CoolMin and CoolMax; otherwise false.</returns>
    private static bool CoolValidation(int cool)
    {
        return cool >= CoolMin && cool <= CoolMax;
    }
    /// <summary>
    /// Raises the CoolChanged event.
    /// </summary>
    /// <param name="e"></param>
    protected virtual void OnCoolChanged(EventArgs e)
    {
        EventHandler eh = CoolChanged;
        if (eh != null)
            eh(this, e);
    }

    #endregion

    #endregion
}