This shows how much time was spent in each heart rate or power zone. This can be shown on a per-day, -week or -month basis.
Double click to zoom in, click and drag to pan, or just use the controls on the chart. Zones can be hidden by clicking on them in the legend. The legend can be hidden (which is useful if it's on top of data you want to see) by clicking on the grey box at the bottom of it.