automatic property

It's called automatic because the compiler automatically expands something like this:

   public int MyProp { get; set; }

into this:

   private int _myInt; // in practice this will be something very obscure

   public int MyProp
   {
      get { return _myInt; }
      set { _myInt = value; }
   }

The private field is not directly accessible (except by using reflection) and so you always need to access it via the automatic property.