What is covariance and contravariance in c# ?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
 
namespace ConsoleApp4
{
 
    //>>Allowing you do things in your code that previously 
    //you where surprised  you could not do.
    //-- Anders Hejlsberg
    //>>covariance and contravariance are introduce in .Net 4.0
    class Animal
    {
        public void Walk()
        {
            Console.WriteLine("Animal can walk");
        }
    }
    class Dog : Animal
    {
        public void Walk()
        {
            Console.WriteLine("Dog can walk");
        }
    }
    class Cat : Animal
    {
        public void Walk()
        {
            Console.WriteLine("Cat can walk");
        }
    }
    class Program
    {
        public static void Main(string[] args)
        {
            Animal obj = new Cat();
            obj = new Cat();
 
            //>>Covariance preserves assignment compatbility 
            //between parent and child
            // relationship during dynamic plymorphism
 
            //>>After .Net 4.0 you can also assign group of 
            //objects from child to parents like below
            IEnumerable<Animal> animals = new List<Dog>();
            animals = new List<Cat>();
 
 
 
            Action<Dog> contravarient = DescribeAnimal;
            contravarient(new Dog());
 
 
            Action<Dog> contravarient2 = new Action<Animal>((Animal animal) => { });
 
            Console.ReadKey();
        }
        static void DescribeAnimal(Animal animal)
        {
            Console.WriteLine(animal.GetType().Name);
 
        }
    }
}