What is the appeal to nature fallacy?

Arguing that because something is "natural" it must be good, right, or better — and "unnatural" things bad — when naturalness doesn't track value.

The pattern: X is natural, therefore X is good/healthy/correct (and the converse for "unnatural"). It fails because "natural" and "good" are different properties: plenty of natural things are harmful (arsenic, venom, deadly diseases) and plenty of artificial ones are beneficial (vaccines, antibiotics, eyeglasses).

"This remedy is all-natural, so it's safer and healthier than the lab-made drug."

Being plant-derived says nothing about whether it's safe or effective — hemlock is natural too. Whether something works has to be shown by evidence, not by where it came from.