A heatmap represents visually how a user interacts with a webpage, by using color codes. The color scale ranges from blue (least interaction) to red (most interaction).
Heatmaps allow you to assess which website elements users click on and scroll through or which they ignore.
Read more on heatmaps in Piwik PRO help center: Piwik PRO site inspector for Chrome
Here is an example of a heatmap: