The Preview Selection area of the timeline is located underneath the timeline
position indicator numbers and is the place to select the portion of the
project to preview. By default the entire project will play when the Quick Preview
button (on the left with three sets of arrows) is clicked. To specify a portion
to preview, left-click on the gray area where the preview should begin and
left-click again to set the ending position. The defined preview range will then be
indicated by a blue bar. To clear the settings, right-click on the gray area.