This is a section dedicated to your top 12 posts and can be rank either by:

ER: Content posted within the selected time period ranked according to engagement rate. Engagement rate is the number of unique users who liked, clicked, commented, or shared the post divided by the post reach. 

Eng: Content posted within the selected time period ranked according to total engagement. Total engagement is the number of likes, comments, shares and clicks. 

Reach: Content posted within the selected time period ranked according to reach. Reach is the number of unique people who received an impression of the post.

For more information on Facebook metrics, please refer to the following FB developer resource: https://developers.facebook.com/docs/graph-api/reference/v20.0/insights