It's a value describing if the system was able to create the dialog or not. The next two parameters speak for themselves, the parent and the owner window handles. Well, this one needs a little explanation. For now we use the default window procedure to handle the messages sent to the dialog. This is one of the most common errors made by programmers and can lead to erratic and uncontrolled behaviour.
Dialogs have a special default procedure.
This procedure is called WinDefDlgProc. If we use this name for pfnDlgProc, the dialog will show and react to our actions. However, it won't do a specific task. For that we've got to program. This is a handle to a resource file.