well... at first you need to read some C++-tutorials.

what you see is the head of a "constructor" with an "initialization list". The constructor has one parameter, a pointer to a QWidget. this parameter is called "parent". It refers to the parent widget or is NULL if you want to create a custom widget. "QDialog(parent)" does not inherit anything. The class "OpenDialog" is inherited from QDialog. That's why the Constructor of QDialog is called. The next line (3) initializes the member variable "ui" with "new Ui::OpenDialog".