Watch dog timer is resource found in the microcontroller that reset the device after a determined time if don't reset it. It's a very useful tool for avoiding system failures. Get a datasheet of any microcontroller it should help u.
Added after 11 minutes:
i attached a pic datasheet
/Deleted. Datasheet for Pic16F628A is available free at microchip site. (klug)/
No all microcontrollers have it. Its kinda must have for programing this kind of devices u should buy a book or get some more data in the net on microcontrollers in order to upgrade ur knowledge.