After completing a meditation session, you'll see the Session Details screen with details about how your body responded to the session.
The chart at the top shows you how calm you were over the course of the session. The shaded blue sections represent time that you were calm, and the dots represent Recoveries, or points during the meditation that you went from being more distracted or stressed, to calm. Tap the chart for more information.
The chart is generated by looking at your heart rate variability, a leading indicator of stress, over the course of the meditation session.