Something unnatural is strange, fake, or abnormal. If you dye your hair hot pink, it's going to look unnatural — especially to your conservative grandparents.
It's unnatural for a wild animal to live inside someone's house; it goes against the way the animal normally lives in its natural environment. You could also describe your bright blue bottle of juice as an unnatural color. And if you try to impress someone by speaking in a French accent, that would also be unnatural. Unnatural adds the "not" prefix un- to natural, which comes from the Latin word naturalis, "by birth," or "according to nature."