A tube skirt is a skirt in the shape of a tube, the same width throughout its length. It is made of stretchy material such as Lycra or a knitted fabric so that it can go over the hips and still not be loose at the waist. As a result, the skirt is very tight over the bottom, emphasising its shape. (It will therefore show VPL if anything is worn under it.)
Tube skirts are often quite short, of miniskirt length. Sometimes they are rather longer, but they tend to ride up while being worn, or can be hitched up and stay in place, so effectively become much shorter. They can also be ankle-length (in which case they act as hobble skirts).