This is a section dedicated to your top 12 posts and can be arranged either by:
Engagement Rate (Percentage of people who Liked, Commented, Shared or Clicked on your post divided by the Post Reach) or alternatively,
Engagement (The total number of times people liked, commented, shared or clicked on your post).
This can be toggled between ER and Engagement via the top right hand corner of the chart area. A graphical representation is shown below: