Impressions is the number of times a post from your Page is displayed. People may see multiple impressions of the same post. For example, if someone sees a Page update in News Feed and then sees that same update when a friend shares it that would count as 2 impressions.
Reach is the number of people who received impressions of a Page post. Reach might be less than impressions since one person can see multiple impressions. For example, if a person sees a Page update in News Feed and then sees that same update when a friend shares it that would represent a reach count of one.
Likes is the number of times people Like your posts. Likes are an indicator of how well your content resonates with people.
Comments is the number of comments people make on your posts. Comments can provide insight into how people feel about your content, products or brand overall.
Shares is the number of times people share your content. Shares measure how much your content is amplified by those who see it.
Engagement is the sum of Likes, Comments and Shares for your content (posts).
Engagement = Likes + Comments + Shares