What does the keyword virtual impictly do to a function ? (How does the compiler consider functions with virtual keyword)

Question

I have visited quite a few links by now.
I seem to understand how to use the keyword virtual.
But I still haven't got an insight as what it does to a function. ( leave alone classes!)

I have a few guesses as what it might do..
Please allow me to explain myself through the following code fragments

Classes

  class stars 
{ 
    public :
        void describe { cout << "Stars are bright objects\n"}
};

  class sun : public stars
{
    public: 
        void describe { cout << "Sun is visibly the most brightest star\n";}
};

Main Function

  void main()
{
    sun *ptr = new sun;   
     stars *spt = ptr;
    ptr -> describe();        //Output line1: "Sun is visibly the most brightest star"  
    spt->describe();          //Output line2: "Stars are bright objects"
    ((sun*)stp) -> describe() //Output line3: "Sun is visibly the most brightest star"
}

Now, had I used virtual before void describe() in class stars. The second line would have been different. I believe it's because, the keyword virtual influences the way the function works.
(i.e) while executing stp -> describe(); the function stars::describe() finds the object type associated with the address stored in stp, and accordingly prioritizes the function to be called.

Please comment, if I didn't make myself clear (this is my first question)
Thank you for sparing your precious time for reading this, I really acknowledge it