What does the virtual keyword do?

#include<iostream>
using namespace std;
 
class parent {
public:
    virtual void virtualDisplayMessage()
    {
        cout << "Parnent class function virtual function."<<endl;
    }
 
    void simpleDisplayMessage()
    {
        cout << "Parent class simple function."<<endl;
    }
};
 
class child : public parent {
public:
    void virtualDisplayMessage()
    {
        cout << "Child class override virtual function. "<<endl;
    }
 
    void simpleDisplayMessage()
    {
        cout << "Child class simple function. "<<endl;
    }
};
 
int main()
{
   // declare class object
    parent *parentClassPointer; // make parent class pointer
    child childClassObject; // make dreived class object
    parentClassPointer = &childClassObject;// assign derived class object
    // calling class fucntions
    parentClassPointer->virtualDisplayMessage(); //  Runtime polymorphism is acheieved using virtual function
    parentClassPointer->simpleDisplayMessage(); // Non virtual function,
   
    return 0;
}