Watchdog timer is basically a timing device that is set for predefined time interval and some event should occur during that time interval else the device generates a time out signal.
One application where it is most widely used is when the mobile phone hangs and no activity takes place,in those cases watchdog timer performs a restart of the system and comes to the rescue of the users.
It is used to reset to the original state whenever some inappropriate events take place such as too many commands being given at the same time or other activities that result in malfunctioning of the GUI.It is usually operated by counter devices.