Watch dog timer is a count-down timer which when reaches a zero count resets the CPU. This is enabled when the controller is entering a critical processing stage where there are chances of hanging or jumping to any arbitrary address. If something goes wrong in this execution stage, after pre-determinded time, watchdog will reset the CPU.
If execution is finished without problems, watch-dog is disabled at the end.
I don't know if 68HC11 has a WDT. Check it out.