This is the 14800th most frequent Russian word.
плоский
Flat.
Here, "плоский" is used to describe a physical attribute referring to an even or flat surface.
У этого стола плоская поверхность.
This table has a flat surface.
In this instance, "плоский" describes the simple, undecorated attribute of the wall's surface.
Картина висела на плоской стене без украшений.
The painting hung on a plain wall without decorations.
Here, "плоский" is used metaphorically to criticize something as lacking depth or creativity.