Impressions are the number of times a post from your Page is displayed, whether the post is clicked or not. People may see multiple impressions of the same post. For example, someone might see a Page update in News Feed once, and then a second time if their friend shares it.Reach is the number of unique people who received impressions of a Page post. Reach might be less than impressions since one person can see multiple impressions.
Tags - Facebook Reach, Facebook Impression, Reach, Impression, Difference between Reach and Impression