Plain wallpaper refers to wallpaper that has a single, solid color or a very minimal design. It is often used as a background or a base for other design elements in a room, such as furniture or artwork.

Plain wallpaper can be used to create a sense of calm and simplicity in a space, and it is often used in minimalistic or contemporary design styles. The color of plain wallpaper can be used to create a specific mood or ambiance in a room, or to complement other colors and patterns in the space.

Plain wallpaper can be made of various materials such as paper, vinyl, and non-woven fabrics. It can also come in different textures such as matte, glossy, or metallic. Plain wallpaper can be used to cover an entire room or as an accent wall, depending on the desired effect.

It's also important to note that plain wallpaper doesn't have to be boring, it can be used to create an interesting effect by combining it with different textures, patterns and colors. It can be used as a background to make other elements in the room stand out, or it can be used to create a focal point by using a strong color.