Polymorphism means one name many forms. Polymorphism means one object behaving as multiple forms. One function behaves in different forms. In other words, "Many forms of a single object is called Polymorphism."
Polymorphism is often expressed as 'one interface, multiple functions'.
Suppose if you are in class room that time you behave like a student, when you are in market at that time you behave like a customer, when you at your home at that time you behave like a son or daughter, Here one person present in different-different behaviors.
In c# Compile time Polymorphism is method and operators overloading. It is also called early binding.
In method overloading method performs the different task at the different input parameters.
Runtime time Polymorphism is done using inheritance and virtual functions. Method overriding is called runtime Polymorphism. It is also called late binding.
When overriding a method, you change the behavior of the method for the derived class.