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 individuals who viewed a piece of content. This is often referred to as "Unique Views" in online marketing. Reach may be less than impressions since a person is only counted once, even if they see a post multiple times.

There are two types of Reach: Organic and Paid. Organic Reach is the number of people you reached just because they follow you or they sought you out. Paid Reach is the number of people who saw your content because you'd paid Facebook to show it to them.