A vegetable noted for its nutrition, grown in various parts of the Disc.
It is notable in that Rincewind was dreaming of potatoes even in a short encounter with some rather attractive Amazons in need of some non-magical services. He appears to use it as point of mental focus, such as when trying to hide his intentions from the Queen of the Elves in The Science of Discworld II: the Globe.
The dietary import of the underground food gained spiritual significance to Mr. Tulip as his sole memory of his village had been to the effect that all would be right if they had potatoes. This became a focus of his in ensuring that he had his potato with him to absolve him of his sins. There is even a God of Potatoes, Epidity.
In The Long Earth, potatoes are elevated to a new level of significance when the humble tuber becomes the power source of a world-changing machine. Some people find they can step between worlds without one. But even so, they still crave potato-based foodstuffs, or find the mundane act of peeling a potato keeps them sane... In our version of Earth the only known potato powered device is a potato clock which works out the energy contained a potato.