What are enums in C#

using System;

namespace Enums
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine($"{Planets.Earth} is the #{(int)Planets.Earth} with radius {(int)PlanetsRadius.Earth}");
        }

        enum Planets
        {
            Mercury = 1,
            Venus = 2,
            Earth = 3,
            Mars = 4,
            Jupiter = 5,
            Saturn = 6,
            Uranus = 7,
            Neptune = 8
        }

        enum PlanetsRadius
        {
            Mercury = 2439,
            Venus = 6051,
            Earth = 6371,
            Mars = 3389,
            Jupiter = 69911,
            Saturn = 58232,
            Uranus = 25362,
            Neptune = 24622
        }
    }
}