The heatmap represents the number of pageviews performed by the contact or all contacts of the account. Each square represents a day and is blue if at least a pageview has been performed (the blue intensity increases with the number of pageviews). You can hover each square to see the number of pageviews performed.
Important: Pageviews are different from Events so the heatmap can be empty even if contacts performed a ton of events but no pageview. Learn the difference between pageviews and events here.