What OOP concept best describes the code below

class program
{
    abstract class animal
    {
        public abstract void eat();
        public void sound()
        {
            Console.WriteLine("dog can sound");
        }
    }
    class dog : animal
    {
        public override void eat() { Console.WriteLine("dog can eat"); }
    }
    static void Main(string[] args)
    {
        dog mydog = new dog();
        animal thePet = mydog;
        thePet.eat();
        mydog.sound();
    }
}