The slider is the highlighted rectangle that appears in
  the scroll bar. The size of the slider indicates how much
  of the data appears on the screen. For example, if the
  slider takes up 10 percent of the scrolling region, only
  10 percent of the data is displayed in the window.

  To move quickly through the data, move the pointer to the
  slider. Press MB1, and drag the slider through the
  scrolling region. When you release MB1, the information
  from the position in the data indicated by the slider is
  displayed in the window.