Does it mean method override?

If a method is private, then it virtually does not exist outside of the class that declared it, including sub classes. You can redefine it because they can't see it; and they'd be entirely different method. Now, if it's protected or public instead, then it would be visible to the sub class and you'd have a problem, enter polymorphism. You can either tell the compiler not to worry about it by declaring it as a new protected void MyMethod() and then the compiler would know what to do. The thing is, any call to the method would be dependant on the type you're calling it from If you're calling it from the parent class, no virtualization takes place, there is no polymorphism in play, the parent class's method will be called. If you're calling it from the sub class, then all's good.

 

And THAT'S where virtual and override come in, now THOSE allow you to call the sub class's method from the parent class