Something unadorned has no decorations or frills. It's plain, like a room with nothing on the walls or a person wearing purely functional clothes and no accessories.
To adorn something is to decorate it or to dress it up. If something is unadorned, it lacks decorations. An unadorned Christmas tree is just a plain old pine tree. If a woman’s face is unadorned, she’s not wearing makeup. The unadorned truth is the plain truth, with no nonsense. This word means about the same as undecorated, and it can often mean dull. But that’s a matter of taste.