A watchdog is a piece of hardware that will automatically restart the chip if it detects the code as being stuck. The way it detects this is that you have to poke a register of the watchdog every so often, and if the register hasn't been poked for a certain timeout (usually configurable), the chip is restarted.

Watchdogs exist on MCUs but also on some "proper" computers. The Raspberry Pi has one for example.