A scroll box is a portion of the scroll bar that helps indicate your location within a document, field, or window. You can also quickly scroll through a page by clicking and dragging the scroll box to where you want to go.
In the picture is an example of a scroll box in the vertical scroll bar of a window. As a page gets longer or has more text, the scroll box gets smaller. On this page that you are reading now because there is not much text the scroll box is either not visible or very big and is more of a scroll bar than a scroll box.