A scrollmap o displacement map is a variation of a heat map or heat map. It shows in color any area of a web that is more visible or how users move through a web portal.
The part of a web that will be seen first is the top of the above the fold. By "fold" we mean the fold line below which any content would be invisible and the area above it is the visible area of a website. Anything above the fold is recognizable at first glance. Everything you see next once you scroll down, that is, pressing the scroll bar down or pressing the down arrow key. These areas generally receive less attention from users. The scrollmap provides graphic information on the scrolling behavior of users highlighting in color the parts of a website that have been viewed. For example, red indicates high activity, yellow, medium activity, and little user interaction blue. Using a scrollmap, webmasters and web analysts can identify which parts of a website are attractive.
Some examples of tools that include displacement maps: 
Importance for usability
Scrollmaps provide important information about the user behavior and they indicate the areas of habitual use of a web. In this way, important content, key information, and call-to-action buttons should be positioned to match the data provided in the scrollmap. Areas in which users spend a lot of time or that receive most of their attention should contain the following: key web items. Scroll maps can also be used in conjunction with eye tracking, treemaps, and other information on user behavior from web analytics to make websites not only easy to use, but also promote conversion. They highlight page impressions, click paths, retention, and bounce rate.