This VOP checks if texture coordinates are defined for the geometry. If so, the those coordinate are returned, otherwise the global s and t are returned. For some geometry, polygons in particular, the global s and t may not give the desired results. A UV Texture SOP will define more desirable U and V texture coordinates.
This is the uvw vector.
This is the U texture coordinate or global s, depending on what is defined.
This is the V texture coordinate or global t, depending on what is defined.