Imagine you have a flat red piece of card in a photographed scene. If the lighting is just right and the card is matt, you could get all the pixels of the card

Imagine you have a flat red piece of card in a photographed scene. If the lighting is just right and the card is matt, you could get all the pixels of the card in the image to be the exact same colour. That would be a single point in the colour space. Now imagine you rolled the card into a cylinder. The result would depend on the lighting, but most likely you'd be able to see some shading across the object, so that there are some darker and lighter pixels. In, the HSV colour space you'd now have a line - all the H and S values for those pixels would be the same, but there would be a range of values for V. Now imagine you replaced the matt card with a shiny piece. You might see a whitish specular highlights where the object reflects a light source or the sky. The added whiteness would have made some pixels have a lower saturation (S). So now you have variation in S and V, but not H. That would form a plane in the HSV plot.