It's the act of rolling a tube, on it's side, across a solid surface (such as a tabletop). A tube is judged by it's speed, consistency and ability to roll straight. Straight-rolling tubes are balanced sounding. Tubes that turn towards the pins (bottom), tend to be bassier. Tubes that turn towards the top tend to place emphasis more on the higher-frequencies. Slow, syrupy tubes will roll slowly, while fast, detailed tubes will roll quick! Perfectly formed tubes (perfectly smooth circle on the rolling axis) will sound pure and clear, while slightly imperfect tubes will be grainy.
Tuberolling is such a great pasttime that there are a few manufacturers who still make tubes today. In fact, many of these current production tubes are good only for rolling across tables. Unfortunately the amount of good, perfectly rolling tubes from the height of the tube-era are fading fast, as they only have a limited-rolling lifetime (accidents do happen, they are made of glass). As they say: "The transistors killed the tuberolling star".
-dd3mon