A scroll map is a variation of a heat map. It displays in color any area of a website which is most looked at or how users scroll through a website.
The first area of a website to be seen is the above the fold area. By “fold” is meant the fold line below which any content would be invisible and the area above it being the visible area of a website. Anything above the fold is recognizable at first glance. Everything below that becomes visible once you scroll down, i.e. pushes the scrollbar down or press the down arrow key. Generally, these areas get less attention from users. The scroll map provides graphic information about the scrolling behavior of users by highlighting in color those parts of a website which have been viewed. For example, red indicates high activity, yellow, average activity and blue little user interaction. By using a scroll map, webmasters and web analysts can identify which parts of a site are attractive.
Some examples of tools that include scroll maps: 
Scroll maps give important insight into user behavior and point out frequently used areas of a website. Important content, key information and call-to-action buttons should therefore be placed in such a way as to match the data provided from the scroll map. The areas where users spend a lot of time or that get most of their attention should contain the key elements of the website. Scroll maps can also be used in conjunction with eye tracking, treemaps and other information about user behavior from Web analytics to make websites not only user friendly, but also promote conversion. Of note are page impressions, click paths, retention and bounce rate.