When a webpage loads, you often can only see part of the page. To see the rest of the page, a user needs to move or scroll down the page to view it. The average scroll is a measurement of how far through a page’s content the visitor went, using percentages of the full page’s length.
The language used here is from newspapers. The content that is visible when the page loads is called ‘above the fold’ and anything that needs to be scrolled down to is ‘below the fold.’ (Think of how you unfold a newspaper to full length, and how it appears when in a pile.)