Разобрать код с Enum

using System;
 
enum Priority
{
    Zero,
    Low,
    Medium,
    Important,
    Critical
};
 
class Program
{
    static void Main()
    {
    // New local variable of the Priority enum type.
    Priority priority = Priority.Zero;  //Что здесь происходит ??
 
    // Set priority to critical on Monday.
    if (DateTime.Today.DayOfWeek == DayOfWeek.Monday)
    {
        priority = Priority.Critical;
    }
 
    // Write this if the priority is important.
    if (IsImportant(priority))
    {
        Console.WriteLine("The problem is important.");
    }
 
    // See if Low priority is important.
    priority = Priority.Low;
    Console.WriteLine(IsImportant(priority));
 
    // See if Important priority is.
    priority = Priority.Important;
    Console.WriteLine(IsImportant(priority));
    }
 
    static bool IsImportant(Priority priority)
    {
    // Switch on the Priority enum.
    switch (priority)
    {
        case Priority.Low:
        case Priority.Medium:
        case Priority.Zero:
        default:
        return false;
        case Priority.Important:
        case Priority.Critical:
        return true;
    }
    }
}

Priority priority = Priority.Zero;

const int Zero = 0;
const int Low = 1;
const int Medium = 2;
const int Important = 3;
const int Critical = 4;
 
int priority = Zero;

public struct Priority
{
    public const int Zero = 0;
    public const int Low = 1;
    public const int Medium = 2;
    public const int Important = 3;
    public const int Critical = 4;
 
    public static implicit operator Int32(Priority p) {
        // ...
    }
 
    // и т.д.
}