Textures and UVs
Textures are pixel maps, usually image files, you use to control surface values such as color, opacity, displacement, and so on.
To map textures to surfaces, Houdini must know which parts of the pixel map should appear on which parts of the surface. You do this by putting UV coordinates attributes on the surface, which map to pixels in the texture. In Houdini, you need to add UVs to all surface types. NURBS surfaces do not automatically have UV coordinates.