A virtual function is a function that is declared in a base class and is redefined (or overridden) in a derived class. When a virtual function is called on an object of the derived class through a pointer or reference of the base class, the derived class's implementation of the function is called instead of the base class's implementation. This is called dynamic polymorphism or late binding.
The importance of virtual functions in C++ lies in their ability to facilitate the implementation of polymorphism. Polymorphism allows us to write code that can work with objects of different classes in a generic way. Virtual functions are a key component of polymorphism because they allow derived classes to provide their own implementation of a function defined in the base class.
Virtual functions are often used in inheritance hierarchies where different classes have different implementations of the same function. By declaring a virtual function in the base class and providing different implementations in the derived classes, we can write code that works with objects of any class in the hierarchy and still get the correct behavior.