A watchdog is a component which expects regular “I’m alive” notifications, and resets the system if it doesn’t receive any over a given period of time. Its purpose is to reboot a system which stops being responsive. Useful watchdogs are implemented as separate components so that they continue “ticking” even if the system’s kernel has locked up, and can thus recover from the hardest of lock-ups.
Whether you need one depends on what you use your system for. If you want your system to reboot automatically if it locks up, then it’s worth enabling; if you always use your system interactively, it might not be so useful.
Look for packages with watchdog
in their names in your favourite distribution; systemd can also take care of this for you (see linux watchdog and systemd watchdog). You might need to enable the hardware in your firmware setup too.