KNotify is the notification subsystem within KDE, which alerts you to events you configure within the KDE system. You can configure KTTS to speak text notifications.
For example, Konversation, a KDE IRC application, permits you to configure a wide variety of notifications. For instance, you can tell Konversation to notify you whenever a message appears in an IRC channel with your nickname in it. Together with KTTS you can have KDE speak the text of the IRC message to you.
In general, if a KDE application supports KNotify, a Notifications menu item will appear under the Settings menu. The specifics for configuring each application with KNotify vary from application to application.
To enable KTTS to speak KNotify text notifications, start kttsmgr, click the General tab, and check the Speak notifications (KNotify) box. Then click the button.