The double strip displayed immediately below the toolbar is called the overview. The overview is a graphical representation of the entire soundfile shown in an active window and displays:
the current position of the cursor (the yellow vertical line)
position of markers - if any (the green vertical lines)
position of click markers - if any (the red vertical lines)
position of a frame - if any (the dark blue frame)
the portion of the material displayed in the active soundfile window (a light blue strip appearing in the upper part of the overview)
the position of the selected block of samples - if any (a gray strip localized in the lower part of the overview)
By placing the mouse pointer anywhere on the overview strip and depressing the left mouse button you can change the position of the cursor along with the content of the active soundfile window. By positioning the cursor on the overview strip and dragging the mouse with the right mouse button held down you can mark a block of samples.