The heatmap allows you to see how much time you spend in particular areas of the field or ground. The redder the lines, the more time you spent in that particular area of the field. This is useful to identify when players are playing in the wrong areas (or the right ones!)
Selecting a particular portion of the game:
You can also move the yellow slider toggles (triangles) below the heatmap to select a particular time span of your performance. For example, this will allow you to view a heatmap of the 1st half in isolation from the rest of the game.
Selecting a particular speed zone:
You can also select to only view heatmap data for particular speed zones. For example you can only show heatmap data for the higher speed zones, which will allow you to see where a player spent their time on the field running. If you have a look at the zone 6 heatmap data in isolation you can even start to see some sprint movements and where they happen.