Being able to substitute objects at runtime is the basis of the Liskov substitution principle. In OOP, if a class inherits from a base class or implements an interface, then it can be referenced as an object of the base class or interface. This is easier to describe with a simple example.

We'll define an interface for an animal and implement two animals, Cat and Dog, as follows:

interface IAnimal
{
     string MakeNoise();
}
class Dog : IAnimal
{
   public string MakeNoise()
     {
        return "Woof";
     }
}
class Cat : IAnimal
{
    public string MakeNoise()
    {
        return "Meouw";
    }
}

Then we can refer to the Cat and Dog as an animal as follows:

var animals = new List<IAnimal> { new Cat(), new Dog() };

foreach(var animal in animals)
{
    Console.Write(animal.MakeNoise());
}