Impressions are used to show the sections of the page that are viewed the most. A section of the page will receive an impression if a visitor stops scrolling and the section is in the viewable area of the browser.
Often the area right below the top of your page will have the most amount of views which results in a white band. The reason the section right below the top of the page is white is because it has received more impressions than the top of the page.
Sections lower on the page can be viewed more than sections above it. A section of the page can only receive an impression if the visitor stops scrolling. If a visitor scrolls past the body of the page and stops at the footer, the body of the page will not receive any impressions but the footer will receive an impression.