could someone please clarify the concept of virtual functions?

I've tried reading textbooks and googling and I just get more confused. I have some idea what it is but not enough to make any sense of it. Basically I have to change all of my display functions in my code to virtual functions. Here is my code:

//specification file phone.h

#include<iostream>

using namespace std;

#ifndef PHONE
#define PHONE

                                                                     
                                                                     
                                                                     
                                             
class Phone
{
  public:
        Phone(string /*name*/,int /*extnum*/);
        Phone();
       void displayInfo() const;
       string getName()const {return name;}
       int getExt() const{return extnum;}
        

  protected:
      string name;
      int extnum;

};

class OfficeInfo: public Phone
{

  public:

         OfficeInfo(string /*name*/,int /*extnum*/,int /*officenum*/);
          OfficeInfo();
         void displayInfo () const;
          int getOfficeNum() const {return officenum;}



  private:

         int officenum;

};

class Email: public Phone
{
   public:
           Email(string /*name*/,int /*extnum*/,string /*email*/);
           Email();

           void displayInfo () const;
           string getEmail () const {return email;}



   private:
      string email;



};

#endif



//implementation file phone.cpp


#include "phone.h"
#include <iostream>

Phone::Phone(string newname, int newextnum)
{


name = newname;
 extnum = newextnum;
}

OfficeInfo::OfficeInfo(string newname, int newextnum, int newofficenum)
           : Phone(newname, newextnum)
{
 officenum = newofficenum;



}

Email::Email(string newname, int newextnum, string newemail)
      : Phone(newname, newextnum)
{
  email = newemail;

}


 void Phone::displayInfo() const
{
 cout<<"name"<<name<<endl;
 cout<<"extenstion"<<extnum<<endl;
}

void OfficeInfo::displayInfo() const
{
 Phone::displayInfo();
 cout<<"office number"<<officenum;

}

 void Email::displayInfo() const
 {
  Phone::displayInfo();
  cout<<"email"<<email;


 }

//client code


#include <iostream>
#include "phone.cpp"

using namespace std;

int main()
{
 Phone objPhone("john",111);
OfficeInfo objOffice("mary",222,5551111);
Email objEmail("sharon",333,"blah@something.com");

objPhone.displayInfo();
objOffice.displayInfo();

objEmail.displayInfo();






 system("pause");
 return 0;

}

I just need to know what a virtual function is in layman's terms so that I have some idea what to do.

Thank you in advance for your help! :)